This is very much a "you can lead a horse to water but you can't make it drink"type of situation. But of course you do your best to at least lead her to water.
The easier thing is convincing her totally that you find her attractive. You can joke, be sincere, get romantic, show her with actions, looks, and words.... In short don't back off, but step it up, until she is saying" I know you think I'm sexy and beautiful, but....."
Then you can play the card that you won't let anyone insult your gf ...... Including your gf. This can be a relationship test--is she more commited to her view of the world, or you? If the former, it could be rocky, so tread carefully. But don't let her insult herself, or not go out, or not wear nice things. If you can afford it, jewelry might be good here....it is reasonable to ask her to wear stuff you have given her, it may well draw compliments, and it makes a statement to the whole world about how you see her.
And finally be part of her taking good care of herself. Encourage vegetables, getting enough sleep, going for walks, limiting salt and sugar to something reasonable, etc. It is a lot easier to feel better about yourself when you actually feel good physically.
But all of that is just leading her to the water, the rest is all her.
Good luck.
10 years