SPOILERS:
That she was fighting herself the entire time to "break out" of her shell. There was no rivalry and it was all in her head. But that wasnt made completely clear until about the last few min.
To me, that was a twist. But thats how I interpreted it.
When she ripped her skin off her finger in the bathroom, and she took a 2nd look, her skin was still there. The way I perceived that was her way of trying to dig underneath her exterior and reach that dark side. A multiple personality type of thing. She couldnt embody the "black swan" being the frigid girl she was her whole life because it involved seduction. Like her nutso mom was saying, "youre sick, this role is taking over you". Or some shit, it aint verbatim but you get it lol.