No sex

Does anyone relate? My boyfriend will beg to have sex and when I say no I don’t want to he gets really upset. I feel bad but I never want to do it and when we do it hurts or burns! Our sex life affects whether he wants to stay with me or not. I’m his first everything so he doesn’t understand that just because we used to do it all the time in the beginning of the relationship (4 years ago), doesn’t mean it’s going to be like that forever. I have friends who rarely want to do it and their boyfriends always want to or vice versa but I’m stuck because it’s annoying the way he acts when I don’t want sex.