Lovato's highly-anticipated "Dancing with the Devil" documentary premiered at the South by Southwest festival Tuesday night, giving viewers an unvarnished look at the singer's struggles and triumphs. In one part, she shares she was declared legally blind in the hospital after her 2018 overdose ... her vision so bad she was unable to see her sister at her bedside.

In an interview with The New York Times, Lovato said it took her two months to regain her vision enough so she could read a book again. The singer says in 'DWTD' she's still unable to drive because of occasional blind spots.

The documentary also details the night of her overdose ... Demi says she was out partying with friends, returned home and called her dealer who allegedly supplied heroin believed to have been laced with fentanyl.

TMZ broke the story ... Lovato was discovered minutes from death by an assistant in June of 2018 in her Hollywood Hills home. The singer spent months in rehab and has had an incredible journey to get back on her feet and performing again.