Philip Seymour Hoffman -- Confided In Alleged Dealer -- I Want to Kick Heroin

Philip Seymour Hoffman desperately told his drug dealer he was trying to kick his ever-growing heroin problem ... TMZ has learned.

An informant who has been working with NYC cops since Hoffman's death says he was privy to conversations between Philip Seymour Hoffman and Robert Vineberg -- one of the 4 people arrested last night in a heroin bust.

The informant -- who was central to cops obtaining a search warrant that led to the arrests -- claims Vineberg regularly sold heroin to Hoffman. He says in some of the text messages Hoffman told Vineberg he was struggling to kick his heroin habit and wanted nothing more than to clean up his life.

The informant was very specific, saying Hoffman bemoaned that the worst part of getting clean was uncontrollable bowel movements.

And according to the informant, he would often see Hoffman in Vineberg's apartment and he seemed like "a man on a mission." There was no small talk ... just business. And he says Hoffman always looked unhappy.