Girlfriend never wants to have sex

So me and my girlfriend have been dating for almost two years, and were both 19. At first it seems like we would have sex and do oral or other forms of pleasure at every chance we got. As of late, we'll have sex or "fool around" (as we call it), once or twice a month if I'm lucky. She'll often say something like "tonight we can" or "on Friday for sure", and then at the last second she says that she doesnt feel like it, not in the mood, is too tired, insert excuse here. She hasn't initiated anything in MONTHS too, it's always me that has to (attempt) to get her in the mood, and more often than not I just eventually give up.

She knows that she isnt fulfilling my sexual desires, because she tells me she wants to be able to fulfill them, and she is 100% tolerant and willing towards any of my fetishes (weve only done vanilla anyway). Were both still very passionate towards each other, and she doesnt feel distant, but when it comes to sex it seems she just doesnt want to, ever.

I think maybe I'm overreacting, but it's really been tearing me apart. It feels like she doesnt want me anymore, and I had having to go rub one out when I get in the mood since she rarely has sex with me. It makes me feel sad, and I hate constantly imagining being with another woman who is able to fulfill my sexual needs, but I would never cheat or anything like that. Maybe I'm just being selfish but I couldn't take it anymore and I needed to get some advice, or some sort of human feedback on my situation. I've already started feeling depressed because it just feels like the peak of our relationship is gone, and that she'll want to have less and less sex as time goes on. I try to make everything how she wants, I constantly ask her if theres anything she would like, or any of her fantasies but she says she doesn't have any. I dont think she would hide something like that from me, since she knows all of my most embarrassing fetishes and whatnot.

I'm I just being a spaz? I would really appreciate any feedback, and I would really like to bring back that part of our relationship. I've tried talking to her but its always the same, how she "wants to be able to".