I had similar issue but opposite.
I didn;t want sex as often as my ex bf wanted so he said he will left me and find another girl to f***. It hurt. I thought he doesn't love me just want sex. I felt guilty all the time, I even wanted to die and I always was so unhappy because I knew he is not satisfied with me. i slept with him when I didn;t want it. He said that I'm not okay.

Maybe you didn;t satisfy her sexually, maybe she has another man, maybe you are not that sweet to her and hurt her somehow. There should be a reason why she doesn't want sex.