I feel like i don't exist sexually anymore.

Me [21m] and my girlfriend [21f] have been dating for 4 years and i love her very much. For the last 2 our sex life has decreased exponentially, rarely once a month. We are going to different cities for college but see eachother every weekend, she never initiates and when i kiss her or hug her she is always "alert" to wether it is an atempt at initiating something sexually and tries to end it fast. I have talked about it with her and she says she doesn't know whats wrong with her and why her libido is the way it is. My mind keeps telling me it is because she isn't attracted to me anymore, at the start of the relationship we set our boundaries straight and she told me porn is a no for her because she considers it cheating and i respect that and never watch any or feel the need to. I just feel like im living a asexual person's lifestyle, the worst of all is i can't blame her, she feels the way she feels and she is entitled to it. The constant rejection makes me resent her even against my own will