Why does she make me doubt myself?

I really like this girl and I honestly believe she likes me back. We've been in the same cl since 1st grade and she knows I like her. Her friends told me to ask her to formal last year after they heard I was thinking about asking her, but I never did out of fear (I know, wimpy). She gives me signs she has feelings for me (I see her glancing at me a lot, etc.). But then I always end up feeling like she doesn't like me, even though she gives me signs she does. I don't find myself very attractive, but I've been told I'm cute by girls and a couple of girls have admitted they liked me. So why do I doubt myself? But more importantly, how can I stop doubting myself?