Long term relationship sex

So I've been with my boyfriend almost 3 years . I still find him very attractive and he says the same. The only problem is there's hardly any affection and he never wants to have sex . I can never finish anyway but that's not the point . He can go weeks without and if I mention it he acts like I'm asking for too much . Usually we have sex twice a month . Before and after my period. It's like a chore for him.  I'm afraid he's bored with me or just takes care of himself all the time . Either that or he has no sex drive at all. We're 25 and i feel like things should be better in that department and it often leaves me feeling unattractive or unwanted . I'm not overweight I take care of myself and try to look sexy. Really messes with my confidence. Anyone else have trouble getting their guy to want to have sex ? This is something that's been bothering me for a long time . Everytime i bring it up it causes a fight or he says I'm asking too much. This is very upsetting . I've tried everything . We've even broken up and gotten back together because of it . At first he tries and then it's back to how it was .