What can I do to overcome my fear of rejection?

I think I might be suffering from an AvPD. I don't really have any friends who I actually trust, I have lived the most part of my life a loner and I have always been terrified by the prospect of any kind of rejection and I'm tired of it. I really like this girl and I really, really would like to ask her out but I can't bring myself to do it because I'm really scared she will say no. Is there anything I could do to make overcoming it easier?