May 27, 2015 07:30 AM

As a young boy, Kevin McEnroe knew his mom, Tatum O’Neal, was struggling with drugs, even if he didn’t understand the details of her heroin addiction. “I would lock myself in my room because it was hard to be around her not acting like herself,” McEnroe tells PEOPLE.

After O’Neal’s 1992 split with tennis great John McEnroe, the long custody battle that ensued over Kevin and his two younger siblings, Sean and Emily, added to the turmoil.

“I had to go to a court-appointed therapist at 10 and I had a hard time speaking about it and I shut down,” says Kevin, 29. “I could understand she was hurting and needed some help. When she wasn’t happy was one thing, but when she started to look unhealthy and thin, it scared me.”

But Kevin says he has found forgiveness while writing his family’s story – his well-received new novel Our Town is a fictional account of the life of O’Neal’s mother, actress Joanna Moore, who struggled with a pill addiction of her own and was briefly married to actor Ryan O’Neal.

“Writing the book about Joanna helped me to understand it wasn’t a choice,” Kevin says. “There’s definitely this sense when you are young, that you’re choosing this over us and I think real forgiveness came when I realized this wasn’t the case. When you’re really in it, there is no way out. But when you’re 11, it feels like that – if you keep on doing this – you are not going to see us, so how could you keep doing this?”

“Real forgiveness came when I learned it doesn’t work that way,” says Kevin, who always remained quite close to his mom despite her decades-long struggle.

“There were years my kids didn’t know if I was going to live or die,” O’Neal tells PEOPLE. “Now I call Kevin every single day and I always say I am making up for lost time.”

Kevin’s own missteps with drugs also led him to a deeper understanding of his mom’s struggles. On July 14, the day he got his book deal for Our Town, he was arrested for buying coke, oxycodone and morphine tablets on a Manhattan street corner. (The cocaine turned out to be baking soda and the charges were eventually dismissed.) Two days later, Kevin went to rehab at the Betty Ford Center for two months.

While in rehab, he finished the novel and came to a better understanding of the difficulties that have long plagued his family. “More than addiction and alcoholism, what was passed down to my mother and me was the inability to believe in ourselves,” he says. “And that pairs very well with alcohol and substance abuse.”

“I came to understand how drugs can make a good person do the wrong thing,” he reflects. “She was Tatum – and then she was someone else when she was doing drugs, so it wasn’t like the decision she was making was reflective of her morals. I thought I had forgiven her but I don’t think I had entirely until I could really feel how hard that can be. She was in very deep. It was only when I went to rehab in Palm Springs that I learned what she went though.”

Today, he and his mom are both clean – and closer than ever. “I’m the proudest mom in the history of the world,” says O’Neal, 51, who has not used heroin since 1999, and who has been clean since a brief cocaine relapse in 2011. “Kevin found a way to find my mother’s voice and to tell her story.”

As O’Neal reflects on what has caused her the most pain over the years, she is as candid as ever. “My biggest regret is ever trying heroin,” she says. “That stole so much of my life, but what I’m most proud of are my sweet children.”

For the full interview with Tatum O’Neal and her son, Kevin McEnroe, pick up a copy of this week’s PEOPLE, on newsstands Friday

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