She does sound like she is definitely in the lower end of interest in sex, perhaps in asexual range (there was a link on MSN yesterday about asexuality, might want to google it). Doesn't mean there is anything physio or psychologically wrong with her. But we can hope, right? Something with a nice, powerful, easy cure.
Quote: When we do have sex, she is willing but there is no real interest or desire, this results in a sex life that isn’t very enjoyable for ether of us.
Can you work on separating your enjoyment from her level of desire or interest? Do something like what AtlDave is doing? Or even simpler without her having to do anything different. Seems like you may have a downward spiralling cycle. She isn't interested. You experience no enjoyment because of her lack of interest. She in turn then experiences even less interest (if that's possible)...and so on and on. Maybe you could whistle or walk around with a silly grin for the day afterward...something that makes her smile too would be particularly good. Start getting some positive feelings paired up with the sex again. I don't think this will change her fundamental nature, but it might help make it all a little less unenjoyable for you . Smiling and laughing are contagious and generally make you feel better overall anyway. Can't hurt, right? Plus, this is something you're actually in control of. Her interest level, you aren't.
I also think that as long as your focus is on her interest, it's going to be you trying to change her. Recipe for frustration for you. Potential source of resentment and resistence from her.
Stay focused on what you can do regarding your own enjoyment given the circumstances as they are. Make that your project while she explores the other stuff under her own motivation. It should be enough of a challenge to keep you occupied, given how things are. Even if there is some physio problem, the cure doesn't mean she's going to turn into someone with more than a somewhat greater interest in sex than she has now. So I think you're still going to have to deal with the issue of how to enjoy the sex with willingness more than interest. (But there is always hope.)