I have no sex drive later in the relationship

I don’t understand why this happens and I’m really desperate. I have a boyfriend, we’ve been together for 6 months but I know the problem is me and not him because I’ve had 3 partners before and it happens EVERY time. In the beginning of the relationship I’m really horny, always having sex/masturbating but then, after like 3-6 months I have literally zero sex drive. I don’t feel horny, don’t masturbate, nothing. I feel really bad because my boyfriend is confused why I suddenly changed so much. I have to imagine scenarios from the beginning of the relationship (where we didn’t know each other that well) in order to get horny- it seems like the person makes me horny until I get comfortable and know everything about him. After a breakup, when i meet someone new, my sex drive goes from 0 to 100. I feel like a slut… I don’t know what to do. I’ve never cheated or had fantasy about doing things with other people, I love my boyfriend, I just can’t bring myself to be horny, even things that I did before now disgust me. Help ?