Girlfriend really shy?

I have a girl who I've been with for a long time now, and even though we've been together for a long time she still seems really shy around me.There was one day that it took her almost a whole day to pluck up the courage to kiss me. When we're getting intimate she hides herself from me or just acts stiff in an effort, I think to try to relax herself..but it doesn't work well. She finally told me after I had gotten upset about not feeling attractive to her that she was embarred and shy. What can I do to help her come out of her shell? When we're just talking she can really get creative about all the things she'd want to try with me but when it comes to it, I think she spends so much time trying to figure out how to do it perfectly that hse doesn't end up doing anything. Anyone got any tips about how I can relax her?