Aaron Carter's family is desperate to get him help because they fear he'll relapse, hurt himself or worse ... TMZ has learned.

Sources close to the Carter family tell us some of Aaron's relatives believe he's suffering a mental breakdown -- particularly because he's been saying things like, "I can't take it anymore" and "I'm losing it." 

We're told the family's concern was behind at least one of the multiple phone calls made to St. Petersburg PD earlier this week. The last caller, on Wednesday night, told cops Aaron was threatening to take Xanax and Klonopin ... and has been talking about suicide for months.

Our sources say the family is also worried he's relapsing by inhaling the aerosol spray used to clean keyboards. The family says this is what sent him to rehab back in 2011.

We're told the family is already exploring its legal options to get Aaron into treatment ... whether he wants it or not.