I think the best way you can help her is remind her constantly she is beautiful. Nicknames like gorgeous or sexy can almost burn in her mind the idea that someone believes she is beautiful. Aside from that if she starts talking about the negatives tell her what you see, specific features that you think are beautiful. Then remind that everyone has imperfections and there are only two things we can do about them. Accept them as imperfect or try to work on them in healthy, but either way you/she have/has to learn to love yourself/herself
my girlfriend is stunningly beautiful, curvy, with beautiful smile, eyes, hair, etc. She see similar things you mentioned as being problem. She hates her stomach and her thighs. I just consistently (like multiple times a day) just casually say things like, "you're beautiful" or call her by one of my pet names for her "gorgeous". It does seem like in the near two years we've been together that she has grown in confidence in herself or at least come to accept some of her "imperfections"