Not feeling it with my boyfriend

I'm so horny all the time and masturbate all the time but when I have sex with my boyfriend which is not very often 1 in 3 months I just don't want to do it with him and just want it to end I just tell him to stop halfway through because I don't like it . Before I was with him I LOVED sex but now I don't love sex with him .hes not bad or anything what do you think girls

Ps sorry it hardly makes sense cba for grammar