Hello,
She seems to have very low self-esteem. Just reassure her again and again that you care a lot abot her and don't have any intentions of hurting her, and leaving her for another woman. Don't show her anything that will make her suspicious.
Don't talk about previous gorgeous girlfriends because that will make things worse. I hope she will overcome this problem because if not, this will affect your relationship in the future.
Well good luck to you and your girlfriend and I wish you both happiness.