Sources connected to her family tell us ... Amanda checked into an L.A.-area rehab facility in January -- the end result of a backslide that we're told started months earlier. You'll recall, Amanda was on the cover of Paper magazine in November.

We're told the pressure of being back in the public eye, plus auditioning in Hollywood again ... was just too much, too soon for Amanda.

She's getting treatment for drug addiction along with mental health issues, according to our sources, who also say she's still getting a ton of support from her parents.

As we reported ... Amanda revealed in the Paper feature her 2006 role in "She's the Man" sent her into a spiral of severe depression. She also blamed a lot of her troubles and infamous moments on drugs. At the time of the article, she said she was 4 years sober.

Bynes was diagnosed with bipolar disorder years ago and has been in and out of mental health facilities multiple times in the past decade. During one of her worst episodes, Amanda set a fire in a neighbor's driveway and nearly burned her dog.

She eventually got treatment and her condition improved greatly. She'd been studying at the Fashion Institute of Design & Merchandising in L.A. up until last year ... when she said she wanted to get back into acting.

Amanda remains under a conservatorship started by her parents, but it's been run by a mental health professional since 2014.