This is a delicate issue and you are right to think this through before saying anything you will later regret.
It is not as uncommon as you might think in relationships. One partner invariably has a higher sex drive than the other which manifests itself after the so called honeymoon period ends. Could be anywhere from a few months to a couple of years into the relationship.
All you can really do is talk to her, tell her how you feel and reassure her that it has nothing to do with how you feel or think about her. You must reassure her that you want to continue in the relationship with her.
Because of her image issues, she may be using frequent sex as a way of reaffirming her own desirability. In which case you may well find that she also does not get as much as you think she does either physically or emotionally from sex and you can both back off together.
Quality not quantity is the way to go and probably the best way of diffusing the anxiety whilst reaching a compromise and strengthening your relationship all at the same time.
Good luck.