Brooke Mueller -- Back in Rehab for 19th Time

Brooke Mueller is back in familiar territory -- rehab -- because she now confesses she has a big problem with a powerful drug.

Sources close to Brooke tell TMZ ... Charlie Sheen's ex-wife #3 says she has addiction issues with Adderall -- a psycho-stimulant commonly used for ADD.

TMZ broke the story ... an unresponsive Brooke was rushed to the hospital last week after OD'ing on drugs. Brooke is privately acknowledging she took way too many Adderall pills and it put her in the danger zone.

Brooke --who checked into rehab tonight -- won't bump into any walls in the middle of the night because she knows the facility very well -- she's been there before. She'll be living there for an undetermined period of time.

Brooke's attorney, Yale Galanter, tells TMZ, "Brooke was uncomfortable with the way Adderall was making her behave."

Galanter also insists Brooke had no illegal drugs in her system when she OD'd last week.

Brooke is on probation in Aspen and she is prohibited from consuming alcohol, but Galanter tells us he's confident the low low level of alcohol in her bloodstream would not result in a probation violation.

Galanter adds, "Brooke continues her lifelong battle with drug addiction by taking these steps herself to prevent a further drug relapse."