Carrey's doing OK but he has ADHD. he was in KickAss II
I really enjoyed him in Man On The Moon, where he portrayed the late Andy Kaufman. Although Andy took his comedy to new levels of weird inscrutabilty, he died tragically young from cancer. Kaufman was upset that people could separate his public persona from his personal life of quiet meditation, even though much of his flamboyant "private" life was a series of staged elaborate gags and hoaxes.
I'm not sure what Belushi's and Chris Farley's mental problem's were but they both OD'd on "speedballs" (injecting heroin or morphine plus cocaine).