Why would a girl be so insecure? And can a girl feel better about herself?

Im 15 so i dont really understand girls at all. But any way my gf is rly insecure about herself but i wont her to know how amazimg she is. Some examples of things she says are in quote:" my boobs are to small"(note she wears a 34a) or "my a$$ is to flat" or her voice and laugh are annoying" but i love all of these things about her and much more. Any way hiw can i help her feel better about herself. Please help