Dale Jr. thought he’d have fun writing a memoir. Then he relived his darkest days.

When Dale Earnhardt Jr. decided he was going to write a tell-all book — about the crashes and the concussions and the silent suffering he endured in the final years of his career as a NASCAR star leading up to his retirement in 2017 — it actually got him pretty excited.

This is going to answer all the questions fans have about why I stopped racing, he thought. And it’s going to help people, others who are suffering in silence with concussion-related symptoms — to let them know they’re not alone, and to hopefully inspire them to seek help from medical experts like the one who helped him.

Writing this book, he convinced himself, is going to be fun.

Well, Earnhardt will know fairly soon whether “Racing to the Finish: My Story” does indeed address his fans’ questions, and to what extent it could help people: The hotly anticipated book is set to be released on Tuesday, six days after his 44th birthday and 539 days removed from the historic announcement of his retirement.

But he can say now, definitively, how much enjoyment he got out of the process of putting the book together:

Virtually none.

“I hated re-living it,” Earnhardt told the Observer in a recent interview. “And I never even thought about that, I guess, when we were starting. ... Then we got down to having to talk about it, and I’m like, ‘I don’t want to talk about it. I don’t want to re-live it ... and think about those symptoms, and remember those dark days, and remember the feelings that I had, and the emotions I had.’

“Having to re-live them just made it all too real again — and that was miserable.”

Though Earnhardt says he suffered close to two dozen concussions over the course of his 20-year career, he points in the opening pages of the book to two injuries that he views as pivotal in shaping everything that was to follow.

The first was a violent crash during a tire test at Kansas Speedway in August 2012, when he hit the wall going 185 mph and stumbled away with a concussion that ultimately would sideline him for two races later in the season. “That crash was the one crash that made it so easy for me to get concussions, and it hurt my brain so badly,” he says.

In fact, that injury and the headlines that were generated when he missed those races led him to seek advanced treatment from Dr. Micky Collins, director of the University of Pittsburgh’s sports medicine concussion program. Earnhardt opened up publicly about his experience with concussions and the importance of appropriate medical care, and he encouraged others to follow his lead.

But then came the second of those pivotal injuries: At the Duck Commander 500 at Texas Motor Speedway in April 2014, he clipped the muddy infield grass and lost control as his car veered right before slamming into the outside retaining wall multiple times.

In the book, he writes that “it was like an old wound had been opened. ... I knew something wasn’t right. I knew it instantly”; the next morning, he secretly started what he calls in the book “a journal of symptoms” in the notes app on his iPhone.

This time, he opened up to no one.

“I didn’t want to go back to the doctor,” Earnhardt says. “My thought process was, I went to the doctor in 2012 with a concussion. He sat me out of the car for two weeks, and I did some exercises ... and they helped me. His whole team went through a bunch of mental and physical exercises, then sent me home with a bunch of homework.

“So next time I got hurt, in my mind, I’m thinking, ‘Hey, I know what to do. I’ve just gotta take it easy, do a little homework, and I’ll be good.’ And I would write down in the notes (for a couple or a few days after crashing), and by Wednesday or Thursday, ‘Hey, I’m a hundred percent. Feeling good. OK, I’ve taken care of it. Problem solved.’ And I’d go back and race again. Three months later, I’d crash and feel sick again.

“I just kept trying to take care of it myself, thinking, ‘Ahh, I’ve got, you know, two to five years left in my career, I’ll just get through it, and I’ll be done.’ I kept trying to tell my sister (Kelley Earnhardt Miller, who is co-owner, vice president and business manager of JR Motorsports), ‘I need an exit plan. What do you guys think about retirement, and when should I do that?’ ... I should have sat down with Kelley and said, ‘Hey, I’ve got a problem. I need to go to the doctor.’ ”

Earnhardt pauses, and sighs.

“But I thought, ‘I’m just gonna manage it myself. If I don’t crash real hard, I should be able to finish my career and be done with it. But it caught up with me.”

Between April 2014 and the summer of 2016, he logged an alarming number of notes chronicling a pattern of crashes and disturbing symptoms that slowly wore him down, until things finally came to a head and he was forced to miss the second half of that season. And even then, he continued jotting down notes in secret.

Collins — who became so trusted by Earnhardt and was so successful in the treatment of his injuries that the star driver eventually enlisted his doctor to pen the foreword to this new book — was among the first to learn of the private journal. Earnhardt eventually also revealed it to Amy, and later to his sister Kelley.

But to some extent, he only showed them to Miller out of frustration.

“I really didn’t have a sense of how bad he was,” Miller says, “because he appeared fine. The things that he was suffering through were not necessarily things that anyone else could see. ... It never occurred to me that this is something that could make him think that it was career-ending. I just didn’t see it that way.”

“She would automatically go into business manager mode,” Earnhardt writes in the book. “If you stop now, you will be leaving this much money on the table . . . your retirement portfolio would look like this . . . your relationship with this sponsor would look like this. I told her that I didn’t want my business manager’s opinion; I wanted to know what my sister thought.”

Finally, he just opened up the notes app and handed her his iPhone. Within months, Dale Earnhardt Jr. announced he was retiring. And within a year, he had decided to share those notes with the whole world.

That decision was solidified over a lunch meeting at Birkdale Village this past February with Ryan McGee, a senior writer for ESPN who covered Earnhardt’s entire career and had been tapped to help the NASCAR star write “Racing to the Finish.”

McGee came in knowing Earnhardt wanted the book to focus on his struggles with concussion-related symptoms, and frankly, he was filled with anxiety about how he was going to piece together the timeline — because there were so many crashes to chronicle and because he suspected it would be tricky for his subject to pin down how he was feeling after each.

Before they’d even looked at the menus, though, Earnhardt said: “Let me send you something real quick.”

He proceeded to Airdrop the entire folder from his iPhone to McGee’s, then excused himself to make a phone call. “In the five minutes he was gone,” McGee says, “I kind of skimmed through those notes, and I knew as soon as I read them that they were gonna be the backbone of the book. When he came back, I said, ‘Dude, this is it. ... Who else has seen these?’”

At that point, Earnhardt recalls, he could still count the people he’d trusted them with on one hand.

But it was already as if a weight had been lifted. “I felt like, ‘Man, I won’t have to walk around with people wondering why I quit,’” Earnhardt says. “You know, they were blaming everything — my wife, my own love for the sport, whatever. But I was like, ‘Now it’ll be out there.’ ‘Hey man, you want to know why? Here it is. It’s gonna be in this book.’”

Still, there was hard work ahead for both men.

It was almost March, and they had a hard deadline of May 1 to finish, since that’s when Amy Earnhardt was due with the couple’s first child. So over the next several weeks, Earnhardt and McGee would meet up — in Earnhardt’s library, in Earnhardt’s treehouse, on Earnhardt’s plane — and talk in painstaking detail about the crashes, the concussions, the notes, the visits to the doctor and so forth.

“It was pretty intense,” McGee says. “I mean, we weren’t dealing with light-hearted stuff. ... We’d spend four hours going back through the most traumatic experiences of his life, and then I’d have to give him a few days. Because it would wear him out.”

McGee even recalls one instance in which — he suspects, at least — the stress of the process overwhelmed Earnhardt: The day after a session in March when particularly painful memories were dredged up, McGee says Earnhardt had an anxiety attack in the garage at Martinsville Speedway, where he’d gone to enjoy the STP 500 as a spectator.

“I remember him coming back and saying, ‘Man, I had this episode where I freaked out and had symptoms, and I had to call Micky (Collins, the concussion doctor) from the garage,’” McGee says. “All I could think was, ‘Oh, damn, I did this to him.’ Because stress and anxiety trigger these things. ... He continues to tell me that I didn’t do it; I know I did it.”

From then on, they started adding breaks to sessions so the two could just hang out as friends and decompress.

Meanwhile, McGee would race home after their talks to write fast and furiously while channeling Earnhardt’s voice, routinely interspersing calls to Collins for help with fleshing out some of the brain science and concussion-related topics that Earnhardt had covered in their conversations.

Barely three months after that lunch meeting at Birkdale, “Racing to the Finish” raced off to HarperCollins publishing firm Thomas Nelson, and when the book is released on Tuesday, Dale Earnhardt Jr. will finally — hopefully — get the catharsis he’s longed for.

But what then? What are his risks going forward? And is he done writing secret notes in his iPhone yet?

Aside from the process of working on the book, Earnhardt says he’s gotten great joy out of the professional opportunities he’s had since retiring, from starring with his wife in a home-improvement show for the DIY Network to working as an analyst for NBC’s NASCAR coverage and as a contributor to NBCSN’s “NASCAR America.”

He did get behind the wheel again for a one-off race on NASCAR’s Xfinity circuit last month, but otherwise he’s kept himself out of harm’s way and has dedicated much of his new-found free time to his Amy and their 5-1/2-month-old daughter, Isla Rose.

Still, he remains adamant that he would absolutely not be retired from racing right now if he hadn’t wrecked during that tire test in Kansas back in 2012: “If just one little thing would have been different in that morning ... none of this would have ever happened,” he says.

But it all did happen; as a result, here this book is. And though it won’t make Earnhardt any more famous than he already is, the future Hall of Famer hopes it shines a bright spotlight on his doctor, Micky Collins.

For years now, Earnhardt has publicly sung Collins’s praises, and the endorsement has had a considerable effect. “At least 1 to 2 to 3 (new) patients a week come to me from wherever, across the world or country,” Collins says, “and basically I ask, ‘How did you find me?,’ and they say, ‘Dale Earnhardt.’ ”

It’s a virtual certainty that Collins will be seeing and hearing even more of that after the book comes out.

What’s less certain is how often Earnhardt will find himself back in Collins’s office. He told the Observer that he hasn’t logged an entry in his journal of symptoms since last year, and when asked whether that indeed means everything is going great with his brain’s health, he doesn’t hesitate: “I haven’t had any problems.”

For his part, Collins laughs when told of this exchange. “Well, I would hope that he’d reach out to me instead of putting it in his iPhone next time.”

As for concerns about the future?

Collins says more research will lead to more definitive answers, but that he is “very comfortable” with Earnhardt’s current status and expects him to continue to do “very well.”

Meanwhile, Earnhardt is trying his best not to worry about potentially serious ills like chronic traumatic encephalopathy, or CTE, a degenerative disease that doctors believe is caused by repeated blows to the head.

And he’s succeeding. Mostly.

“If you’re gonna write a book about concussions, you’re gonna have to talk about it, you’re gonna have to think about it, you’re gonna have to be connected to it, and discuss it, and be willing to discuss it,” says Earnhardt, who two years ago pledged his brain to the Boston-based Concussion Legacy Foundation, a well-known brain-trauma research and advocacy nonprofit.

“But ...”

Earnhardt exhales sharply and pauses again to collect his thoughts.

“I don’t know, I mean, do you want to wake up every day thinking and wondering about what your future is going to be like, or do you leave it in the hands of fate, so to speak, and enjoy the now, and enjoy what you have, and what’s happening to you today, and trust in the fact that there’s a doctor somewhere every minute of every hour of every day trying to find the answers to problems that you may have one day? I’m hopeful that whatever — if anything — I ever suffer from, that by that moment and by that time, there will be some answers for me.

“Until then, I’m just going to enjoy what’s happening to me in my life.”