if she's up for it, you could try helping her get therapy. i know that a lot of people might cringe at the thought (if you do, or she does, i understand.) it's an idea tho'.
you could start on an exercise regime and invite her to join you. she will probably lose water-weight quickly (the excess water in fat that burns off quickly with exercise) so it might be a confidence booster that could put her on a healthier road to losing the weight she doesn't like. if she has problems with hurting herself, she might be able to redirect her frustration into physical activity.
you could try taking her places and giving her lots of attention (in response to a similar problem someone before mentioned taking pictures of the girl at the park) something to prove to her that you love her and do think she is attractive.

ultimately, it may be difficult and even embarrassing for you, but if you love her, it will be worth it.