How can I help make my girlfriend more confident with herself?

My girlfriend has always had problems accepting herself as being 'good enough' as she puts it. She doesn't view herself as being smart, beautiful, or sexy. Sometimes it seems to get through when I try and compliment her on her looks or intelligence, but when times like these come around, it sounds like she doesn't believe me one bit, regardless of how sincere I'm being.

Recently, I was with her a group of her friends when one of them, for reasons that I still don't know, flashed her boobs (or at least her cleavage, she was still wearing a bra) at several of the people around her. Me not realizing that she would be doing anything and her not realizing that I was looking, I looked over half a second before she did it and looked away as soon as possible. Needless to say my girlfriend was upset but seemed to get over it when I try and explain that I didn't mean to look and that I didn't see anything (I didn't), and she explained that she didn't mean to and didn't know that I was looking. She forgot about it about 20 minutes later and we got on with the evening.

However, today she brought up the subject again, asking me what I really saw (over text). I said again that I didn't see anything, and she is 'making herself believe me'. However, she is now comparing her friend to her, since her friend's breasts are quite a bit larger than her own [D or DD to a size A]. Regardless of what I say, whether it be that I don't care that her breasts are small, that I don't like big breasts (which I don't, I actually find too big of breasts to be a turn off, especially her size), that I like the size of her breasts, or that I love her for who she is, she won't get over it. She's even talking about getting breast implants now when she has the money, which I can not stress how much I don't want that. What can I do to salvage this situation, maker her feel more comfortable with herself, anything at all that will make this better.
How can I help make my girlfriend more confident with herself?
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