How can I solve this libido issue?

My girlfriend and I have been dating for a few years and until recently we were having regularly and everything was great. For the past year, the amount of times we're doing it has steadily decreased to the point of going 3-4 weeks without. Now, she is saying that she never feels like it at all. She says it is not me, and that she is just so stressed that she never feels "in the mood", but I feel like there has to be something I can do to remedy the situation. Any ideas out there?