My girlfriend always rejects sex?

When I try to have sex with my girlfriend she always rejects me. We used to have sex a lot back when we started dating and now every time I want to have sex I get rejected, it’s really bothering me and it got to the point where I want to cheat. We go months without having sex. When we do have sex she says in the middle it hurts and doesn’t let me finish then I wait for a month more. I can’t cheat I wasn’t raised to cheat. I feel like she doesn’t find me attractive anymore or doesn’t like having sex with me anymore. I don't know what to do
My girlfriend always rejects sex?
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