John Fuller, a consultant known for his high-profile clients -- he prepared Martha Stewart for her stay in the can -- tells TMZ ... the rap mogul may not be suicidal in his early days locked up, but concern that Diddy's mental health might take a decline over time is valid ... especially if the trial ends up not going in his favor.

As Fuller put it ... Diddy is likely used to getting his way given his A-list status ... however, those days are over, at least for now -- and that may become complicated for him to process.

Fuller added ... "This is his version of 9/11."

Fuller says a feeling of "complete hopelessness" is entirely possible given the severity of Diddy's situation ... the Bad Boys Records founder has been charged with sex trafficking, racketeering and transportation to engage in prostitution, and he's being held without bond.

His case has been compared to other high-profile sex crime prosecutions, such as R. Kelly's and Jeffrey Epstein's.

Yet, Fuller noted Diddy may be experiencing one perk behind bars ... as John says, celebrities are used to being completely busy -- phones always ringing, meetings, public appearances -- and so, prison could allow for some peace for Diddy.

Diddy is currently being held at MDC Brooklyn -- a federal detention facility in NYC -- after losing his bail appeal hearing.

As TMZ previously reported ... for Diddy's safety, the rapper has been kept in a Special Housing Unit, away from the general population. He likely could be a target in prison, given his celebrity stature and the nature of the allegations against him.

His day-to-day life is now seriously regimented ... getting instructions on when to wake up, eat, sleep, and shower.

We caught up with Diddy's lawyer Marc Agnifilo outside the courthouse, where he expressed his concern about the poor conditions of the facility ... revealing he's trying to get his client moved to a different prison.

All in all, sounds like Diddy is in for a rude awakening behind bars.