Yikes, that's rough. Maybe some kind of couple's sex counseling? At the very least she needs to work with you to TRY to enliven things... even if it may be a lost cause she clearly has a libido and it's not fair for her to just accept that this is all she can give you forever without even trying to make it better, which Is what it sounds like you're describing.
I wish I had better advice. Lacking that, have this funny comic on the topic of spicing things up over the course of a marriage: http://www.smbc-comics.com/index.php...&id=2545#comic
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.