Philip Seymour Hoffman -- 'I Know I'm Gonna Die'

Philip Seymour Hoffman was on a heroin binge 6 weeks before he died, and told friends he feared he was destined to fatally OD ... TMZ has learned.

Sources Hoffman confided in tell us ... back in December Hoffman told them he had started injecting himself with heroin and couldn't kick it. Hoffman said he would kick it for a few days and then fall off the wagon.

We're told he went back to AA in a desperate attempt to clean up ... but much to his great frustration it didn't work.

During the last 6 weeks he often looked disheveled and dirty. At one point someone asked him how bad his problem was, and he responded, "If I don't stop I know I'm gonna die."

We're told Hoffman was also drinking excessively ... which opened the door to many bad decisions when it came to heroin use.

Ironically and sadly, we're told Hoffman went to AA meetings over the last 4 years and was considered a "guru," because he had been sober for so long. Hoffman would give inspirational talks to people who attended.