Hi,
I think, it might be very useful for you to first figure out, if she is Doing it cuz she does not love you enough, or that she really has a phisical problem.
Love is a Two-Way street. Just because you love her a lot, she might not reciprocate the same feeling. If that's the case then the best thing might be to move out. And beyond a point there is no way for u to sort it out, except by talking it out. If u think it's a phisical problem, then being more supportive, and helping her seek help is the best course of action. a certain difference in sexual appetite is normal i suppose, but being non-cooperative or selfish on a routine basis is cause for concern ! Hope you guys work things out. Do keep us posted.
Good Luck.