Well obviously theres something extremely wrong here. Maybe she needs the reasurrance that she is worth wanting etc. I agree with red... I wouldnt put up with 6yrs of not having sex or expect neone else to either! I really think you deserve a medal mate.
I used to HATE sleeping with my ex as he was a total ******... and made me feel like i was just being used for sex.
Or maybe she doesnt enjoy sex with you. I think you need to sit down and really talk to her. Dig really deep. But show her support and act like you really deeply care.
Theres some excellent books that have helped me with relationship issues...
http://www.relate.org.uk/RelateBooks.asp?cat=1 76
Good luck x