I've lost the desire for sex

I'm M, and I really need help/advice cause I honestly don't know what to do. I'm 24 and I'm at the verge of loosing my relationship that's of I haven't lost it already.

The problem here is I feel pain during sex, it's nothing to look forward to, I hear people have pleasure and all, but what I feel is pain, discomfort and at times anger. I wasn't abused in anyway whilst growing up I only realized when I was 22 that I don't really fancy sex.

As it stands now, my boyfriend told me yesterday over the phone that he can't cope anymore, cause I only desire him when his away, that's as a result of missing him but when once the sex begins I feel nothing.

As I'm typing this we don't talk anymore cause he said he prefers someone that he is sexually compatible with.

I keep on asking/wondering what my offence is, I honestly don't know why I detest sex, and I've been in tears cause he is a good person. I honestly don't know what to do