Diddy -- Coach Alosi Psychologically Damaged My Son

The confrontation that led to Diddy's arrest was years in the making, because coach Sal Alosi went on a campaign to emotionally tear down Justin Combs ... this according to sources with inside knowledge.

Our sources claim strength and conditioning coach Alosi engaged in psychological warfare against Justin for the past 3 years. Our sources say Alosi was verbally abusive to the extreme -- publicly humiliating Diddy's son on numerous occasions.

The sources say it was evident to anyone who watched ... Alosi was bullying Justin, and psychologically damaged him.

We're told Diddy was well aware of the "bullying" and when Justin came home Monday, he said he was told to leave the field and not return for the summer. Diddy decided it was time to have a talk with the coach -- and we know how that ended.

We've reached out to UCLA Athletic Dept. for comment ... so far, no word back.