Too much into the past

Ladies, how do I stop caring about my boyfriend's past? It's pretty much normal just as mine is, and he's not had that many affairs nor any TOO serious relationship but I keep thinking of how he had it before me. Do people from the past ever cross his mind? Did he like sex better with anyone else? I have no reason of thinking like that but it's just that it comes naturally. I also keep asking questions about it, even though I know I'd rather not hear the answers, and then I get mad...