All I can do is provide some insights. How you handle it remains a very personal decision between you and your girlfriend. What I can tell you is that it is very rare for two people to be perfectly matched in their level of sexual desire. People have different tastes and appetites when it comes to sex, just as they do with food. This need not be an irreconcilable difference, but it can be.
Couples compromise on all sorts of things.
As long as you don't feel used or coerced, many individuals are quite pleased to do things to please their partner. On the other hand, your girlfriend's expectations may not be realistic. This is not always possible and doesn't mean there is something horribly wrong in the relationship. Even in the best of relationships, the intensity of sex can wax and wane, especially over time. Something else I suspect is that this difference between you two is not as great as it might first seem.