so, my boyfriend and i have been dating for half a year. but i just can't get over his past relationships with girls. it literally eats away at me and all i can't think about is him having sex with them. all the time. when we're having sex, i think about it, etc. i don't know what to do or how to stop feeling this way. i feel like i can't enjoy our relationship because his past always sneaks up on me and makes me feel crappy. help.