Diddy's attorney, Marc Agnifilo, appears in a new TMZ Studios documentary -- "The Downfall of Diddy: The Indictment" -- which has just dropped on Tubi ... and we asked him about his client's state of mind in jail.

Agnifilo says Diddy's remaining remarkably positive and isn't letting the indictment weaken his resolve. Instead, Agnifilo says Diddy is so focused on his defense, it's actually giving him a form of strength and confidence that will help him deal with being behind bars for an extended period.

Diddy's currently locked up in the Metropolitan Detention Center in Brooklyn ... and his old nemesis Suge Knight is warning Diddy to watch his back around the other inmates.

Thing is ... Diddy's still in the Special Housing Unit, which keeps him out of the prison's general population.

Diddy's been put on suicide watch even though his legal team says he's not suicidal ... and here, Agnifilo tells us Diddy's come to terms with the fact MDC Brooklyn is his new home for now and is living by the adage, "Don't tell me what I did wrong, tell me what I do next."

Our documentary chronicles Diddy's indictment, defense, and a ton of issues that have not yet surfaced.