Chris Bosh Reflects on Sudden End of NBA Career as He Releases First Book: It 'Was Painful'

The end came a lot sooner for Chris Bosh than he ever expected.

During the 2015 NBA season, the iconic Miami Heat star began experiencing blood clot problems — the first had traveled from a calf contusion to his lung. When the clots recurred and proved life-threatening, the Heat waived Bosh in 2017. Bosh officially retired two years later.

"My last game was on a random Wednesday night against the [San Antonio Spurs] in Miami," he reflects to PEOPLE. "I thought I had so much more runway ahead of me, as far as playing basketball is concerned."

Learning to deal with the sudden — and major — life change is something Bosh, 37, explores in his new book, Letters to a Young Athlete, published by Penguin Press and out Tuesday. Bosh says he "wanted to write something that could help people rise to the occasion when the challenge comes."

The challenge, for him, was in changing his expectations after facing a sudden health crisis. His feelings at the time are something he revisited while writing the book — and it wasn't always easy: "The tough part I can say was the career coming to an immediate end. ... I had to move on to the next thing and that part was painful."

"I knew I could have done more. I was kind of, not in the twilight of my career yet, but kind of in the third reinvention of myself, and I was very much so looking forward to going through that process of figuring out how to hold on while the young guys are coming up."

Bosh says he knew early on that basketball was his path. After playing a game in elementary school, he recalls his peers and the adults in his life telling him he was an above-average player. He realized how happy the compliments made him — excelling at basketball mattered to Bosh.

Now an 11-time NBA All-Star, Bosh has learned that greatness doesn't come easy. It's been about, as he puts it, "punching in the clock every day."

And that's what he had been doing up until 2017. With the basketball "fraction" of his story over, Bosh is now putting in the work in a different area: finding what else makes him happy. "That was this question that I was faced with," he says. "You know, of course, my career came to an abrupt end, but I had to answer that question."

In addition to cherishing time with his family — wife Adrienne Williams and his five children — Bosh discovered his affinity for writing. Bosh tells PEOPLE he had put pen to paper before, frequently journaling to process his feelings. That transitioned into writing down reflections on his past experiences, with no greater plan. But yet, "It eventually got to the point where, you know, we felt that we had something."

"We've worked extremely hard on writing and putting this book together, we've literally put our blood, sweat and tears into it," Bosh reveals. "After not being able to play basketball just suddenly and immediately having my career kind of come to an end, I found myself doing a lot of reflecting and I just wanted to make something that was sort of a gift to everyone, not only those who have helped me in my path but those who have upcoming challenges that they're going to face to the people that are trying to be great."

Letters to a Young Athlete is part memoir, part advice book, coupling Bosh's career reflections with lessons learned about life skills through his sport and career. Bosh hopes readers will take inspiration from his words in their own journeys to move beyond hesitance, fear, exhaustion, and ego.

And he hopes it helps his fans realize that despite an unexpected end, he has no regrets about how his "act two" began.

"Looking back on it, it was a legendary hell of a ride, I couldn't ask for much more," Bosh tells PEOPLE.

Letters to a Young Athlete is available now.