Is this a lack of self esteem? Or is it normal for girls to feel this way?

I'm seeing this girl. We are both 20 and she is afraid to let herself like me. She said that no one can possibly like her (even though she is beautiful and so smart). She's never had a boyfriend but she told me she slept with over 30 guys. That was kind of hard to deal with but she's worth it. For the past year we spent so much time together. I love her so much and I told her. She loves me too but she said she's afraid to let me into her heart. Why is she so worried? She's perfect to me and she doesn't believe it at all. I don't want our relationship to change, I just told her how I feel.