The Words actor Bradley Cooper, 37, opened up to the Hollywood Reporter about his decision quit drugs and alcohol at age 29.
Cooper, who was named People's Sexiest Man Alive for 2011, said he spent his twenties feeling insecure and alone.
He explained: "I was so concerned what you thought of me, how I was coming across, how I would survive the day. I always felt like an outsider."
He added: "I just lived in my head. I realised I wasn't going to live up to my potential, and that scared the hell out of me.
"I thought, 'Wow, I'm actually gonna ruin my life. I'm really gonna ruin it.'"
A former heavy user of drugs and alcohol, Cooper said he remembers one incident that left a big impression on him.
He recalled: "I was at a party, and deliberately bashed my head on the concrete floor. Like, 'Hey, look how tough I am!'"
The actor said he was bleeding, but 'did it again' and 'spent the night at St. Vincent's Hospital with a sock of ice, waiting for them to stitch me up'.
That incident, along with others like it, led the Limitless star to give up his dangerous lifestyle.
He said: "I don't drink or do drugs anymore. Being sober helps a great deal..."I remember looking at my life, my apartment, my dogs [when I was still using], and I thought, 'What's happening?'"