Faye Dunaway's owning up to her rep for being a bit of a handful ... but she says there was more to it than just her being difficult, 'cause she was actually dealing with bipolar disorder.

The "Bonnie and Clyde" legend dives deep into her diagnosis in a new Cannes Film Festival doc about her life called "Faye" which premiered Wed -- and she reveals a team of docs really dug into her behavior, hooked her up with some meds and it helped her a lot.

Faye says she finally understood the reason for the way she was ... adding, "It’s something you need to be aware of, you need to try to do the right thing to take care of it.”

But, as for those claims of her alleged nightmare-ish behavior at the height of her career, Faye acknowledges, "I don’t mean to make an excuse about it, I am still responsible for my actions."

If you aren't familiar, Faye's past behavior even ruffled feathers with fellow icon Bette Davis, who famously told Johnny Carson working with Dunaway was impossible and uncooperative.

But, back to her Laurent Bouzereau-directed doc ... Faye says her diagnosis is just part of her make-up now and a sensitivity she's learned to navigate -- but she's super grateful to all the doctors, meds and research surrounding the whole thing in the modern era.

FD's son also chimes in, describing his mom's journey ... saying she "started out as a normal person wanting to be famous, and ended up as a famous person wanting to be normal."