There is nothing wrong with you, don't do that to yourself. Some people need to be really comfortable with the idea of having sex in order to relax enough for an orgasm. This can be hard for a woman in the beginning, even if she loves him.
Hormones have a lot to do with it too. I have a hard time orgasming with my bf a few days before my period. I went on birth control but it changed my hormones and I could not orgasm, ever. So we quit and I am getting a diaphragm now instead.
My point is, it is probably not you. It could be an emotional or physical thing. Just let her know it is o.K and maybe ask he if she wants to see a doctor and discuss the issue. Let her know you want to make her happy. Good luck...