This COULD be caused by a mental block. If you feel any kind of nervousness when you are having sex with her that can make a big difference. Maybe you have been so eager to please her that you have worried too much about preformance instead of your own pleasure. It could also be that you "self-satisfy" (everyone does it...relax) too much and dont have as much "mojo" when you are with her because you are already satisfied. We are always good to ourselves because we know what we like. Remember... It's not just about the sex itself that satisfies, it's the way you touch one another and how it feels while you are having sex.
Good luck and thanks for posting City!