Maybe you want to focus your attentions on other things besides sex. If the whole relationship is founded on bedroom antics, it ain't much to begin with. Do you, two, just like being together? I mean, with clothes on, doing something together, hanging out together, things that don't include sex. If every activity you do with her is expected to end up with your clothes on the floor, she might just be getting tired of being 'used'. Maybe she needs to be reassured that you're sticking with her for all reasons, not just for getting your rocks off. Hell! You can do that by hand, for gosh sakes!
What I'm trying to say is: Re examine your motives for being with her. If it always comes down to sex, walk away. Women want more than to lie on their backs all the time. They want to be loved and cared for because you love and care for them.