Aaron Carter Missed Rehab Therapy Session Night Before He Was Found Dead

Aaron Carter was in and out of rehab therapy sessions up until his death, and we're told his last scheduled appointment -- which he missed -- was supposed to happen Friday night.

Sources close to Melanie Martin, Aaron's fiancée, tell TMZ ... she was desperately trying to get him help during their off-and-on relationship -- although he mostly kept her in the dark about any recent addiction struggles.

Our sources say Melanie signed up Aaron for outpatient rehab after confronting him back in September. As we reported, he got a welfare check around that time when fans got worried he was huffing compressed air off-camera.

We're told Aaron agreed to go through the program, with the idea that it would go a long way in helping him regain some custody of their son, Prince -- who's currently under the care of Melanie's mom.

Sources say he stopped the program briefly, but was reinstated last week -- Aaron was supposed to have an online session Friday evening, but was a no-show. When the facility reached out to tell Melanie, she broke the tragic news to them.

Keep in mind, it's possible -- if not likely -- Aaron missed that appointment because he'd already died.

TMZ broke the story ... cops were called to Aaron's Lancaster, CA home Saturday morning for a report of a drowning -- the housekeeper found his body in the bathtub, and the last known contact with him was 2 AM Friday. Aaron's cause of death is currently deferred by the L.A. County Coroner's Office until toxicology results are returned.

Law enforcement sources tell TMZ there were multiple cans of compressed air in his bathroom and bedroom ... as well as prescription pills.