"I didn't 'buy' it- because of the whole ridiculous 'go home and touch yourself' comment. As it was it seemed to reduce her psychosis to sexual exploration, which annoyed me."
Ah you see, it turns out that you didn't get it after all 
Her psychosis was related to the extreme pressure she was under, not the sexual exploration aspect which was only a small part of the movie. She really wanted the role and obviously was pushed & primed all her life to desire it by her mum, but was nowhere near ready to deal with the stress that getting it, keeping it, and performing under it would bring. As a result, she cracked.
'Go home and touch yourself' was a jackass type of comment from the narcissistic choreographer who thought she needed to loosen up and bring out her passionate side. Break through the "nice girl" persona, if you will. A bit of masturbation didn't cause her psychosis, and her psychosis was not reduced to sexual exploration.
"I did think it was exploitative."
Exploitive would have been for him to take her to bed, which would have been easy enough. Was he a nice guy? No. Did he exploit her? I wouldn't say so.
In any case, would you write off a film if a main character is exploitative? Does that mean the film is "shite"?