I agree with >>2 and >>4. I would also like to add that she wouldn't be going out with you if she thought that you weren't good enough for her, and if for some reason she did, then you're obviously doing something right. It might feel natural to feel that way, but you should talk to her about it, but ONLY that you feel that you're not good enough for her. Don't tell her that she should go out with someone else. She'll probably tell you what I just said, but if it keeps plaguing you then tell her.

P.S. B is a pretty good grade to get.