Hamilton, whose band The Wondergirls also performs the song "Let's Go All the Way" on the soundtrack with Robbie Williams, says he's "really grateful for somebody giving me an opportunity again. I just didn't have that before."
The 38-year-old son of George Hamilton has had more than his fair share of troubles when it came to drug and alcohol abuse.
After staying sober for more than six years and getting his career back on track, Hamilton found that he still had one serious problem that needed attention.
"I suffered from bulimia and anorexia," he tells PEOPLE, speaking publicly for the first time about the eating disorder he's had since he was a teenager. "I believe it was my problem before I got into drugs and alcohol. I used the drugs and alcohol to control the food addiction."
Still StrugglesWithin the past year Hamilton sought treatment for his disorder, which he admits he still struggles with.
"I don't have it perfect now. I've been free from bulimia for years but I still struggle with food, restricting or overeating. That's been the hardest for me in sobriety," he admits, noting that during his teen years at Beverly Hills High School his nickname was Fatley.
"When I speak at meetings I have people come up to me and say I suffer from it, too, but I'm too afraid to talk about it because it's so painful. It's almost like drug addiction is totally acceptable to talk about in Hollywood. But food addiction? Nobody wants to talk about that. It's really shameful as a man to have that."
Courtesy Ashley Hamilton
Treatment ChallengesAt times, the 6-foot-3 actor says his weight fluctuated from 180 pounds to 250 pounds. And even now, after he's sought help, Hamilton says he's not quite at 100 percent yet.
"With food it's not black and white. It's: 'Oh, my behavior around food has been better this month,' " he explains. "You can't live without food. You're surrounded by food constantly and you have to eat. There are so many people in our country suffering from this. It's an emotional problem and that's been the struggle for me through sobriety."
Hamilton's longtime friendship with Iron Man star Robert Downey Jr, who had his own longtime struggles, "helped open the door" to Hamilton's work on the movie.
"But he said 'I can't do it for you, Dude,'" Hamilton explains. "I said, 'That's ok. I just need somebody to give me the opportunity to trust in me sober-wise and give me a shot of this magnitude.' I'll forever owe him for that."
Now, says Hamilton, "I'm just more in a place of gratitude." He's currently working on the feature film Tranzloco, and a play.
"When I was younger it all came so easy and I didn't appreciate it," he says. "I didn't feel like I'd earned it, you know? Now I feel like I've earned it and I've really worked for it."