[Sex Drive] My (23M) girlfriend (21F) can have a very low sex drive and I don't know what to do.

So, I've been with my girlfriend now for over two years (lost my virginity to her, somewhat ironically two years ago today) and I'm really confused about how to tackle this issue. She's on the pill (rigevidon) and I'm aware this can play havoc with a woman's sex drive but when she hits her low points there's nothing I can do to get her going; even if I mention how we barely have sex to try and talk about it she just gets angry. I try to be caring and attentive to her by intiating with oral or just my hands with the hope some of it comes back my way but more often than not she'll just have an orgasm and say she wants nothing else, nor will she do anything to me (tends to be a very selfish lover). That's if she'll even let me touch her, most of the time she'll shrug me off and say she's not in the mood.

Currently it's 3 weeks since we last had sex (one of them was her period week) and it's difficult for me but the question I have is: am I being selfish? I mean it's causing me to look and fantasise about other women and that seems like a bad road to be honest. I wouldn't ever want to hurt her like that but it just seems to me like we're young and we live together, surely we should be having sex more regularly? Even when she's in the mood it's like twice a week, which I can deal with (I have a reasonably high drive everyday would suit me) but lately it's been nothing.

I've tried bringing it up with her before but the only thing I've ever gotten from her is 'I just don't need to have sex that much' is it normal for a woman (who definitely doesn't masturbate) to go so long without needing it? I'm trying to be patient and have done in the past but it's beginning to grate on me a bit.

So what would you chaps do? Do any females with a low drive anything for me also?

Thanks.

TL;DR

Gf has low sex drive while I have a high one, can't get a reasonable response from her and it's causing frustration, need to resolve it before it causes further problems.