Selena Gomez -- BAILED On Rehab

Selena Gomez ignored the advice of medical professionals and waltzed out of rehab just 14 days into a 6-week program ... over the strenuous objections of the rehab staff ... TMZ has learned.

Sources close to Selena tell us just after New Year's she checked into a program at The Meadows called DAWN -- for alcohol, pot and Rx Ambien -- in addition to what her people believe is an unhealthy union with Justin Bieber.

But 2 weeks in, Selena bailed so she could go the Sundance Film Festival for her upcoming movie. She told her people she would return to the rehab facility after Sundance, but when it was over Selena decided she was cured and there was no reason to return.

Problem is ... that's not the way the medical staff at The Meadows sees it ... the treatment program is 45-days and Selena bailed four weeks early. We're told the staff feels it's essential in any recovery to complete the therapy.

Selena is telling her people ... she completed the toughest week, which the facility calls "Survivor's week" so everything is fine. Thing is ... Selena's not the doctor -- she's the one with the problem.