Pitt, star of Moneyball, for which is nominated for Best Actor Oscar, said he got "really sick" of himself in the 1990s, when he was smoking too much marijuana.
"I was hiding out from the celebrity thing, I was smoking way too much dope, I was sitting on the couch and just turning into a doughnut and I really got irritated with myself," he told The Hollywood Reporter.
"I got to: 'what's the point? I know better than this'."
He continued: "I used to deal with depression, but I don't now, not this decade – maybe last decade. But that's also figuring out who you are.
"I see it as a great education, as one of the seasons or a semester – this semester I was majoring in depression.
"I was doing the same thing every night and numbing myself to sleep, the same routine – couldn't wait to get home and hide out.
"But that feeling of unease was growing and one night I just said; 'this is a waste'."
Pitt, whose long-term girlfriend, Angelina Jolie, said he pulled himself out of his funk after seeing extreme poverty while on a trip to Casablanca.
The Fight Club actor was commended for speaking so candidly about his experience.