Having a lot of trouble w/past sex/relationships, please help!

Hey ladies, I have an issue that I feel like I'm the only one in the world who seems to have/care about. My current boyfriend is the only guy I've had intimate sex with, he's the only one to have been inside of me/be one with me, to finish with me and make me finish, to hold and hug me afterwards with all those hormones of love and affection and intimacy just swarming around us, to embrace each other passionately, etc. But I am not his first. In fact, I'm his 6th. And one woman he slept with for 2 years. I get these horrible intrusive thoughts sometimes, of imagining him with these other women, especially the one he was with for 2 years. He must have experienced that passion and fire with them like he and I do, and embraced them, been inside of them, and breathed hard the way we do, say the same passionate words to each other in the heat of the moment the way we do, he must have shared this incredible intimate moment with them the way we do. And it TEARS me apart inside. I don't know why, but it eats at me so much. When they pop in my head, I feel like punching into my chest and ripping my heart out. Sometimes these thoughts come to me WHILE he and I are having sex. Sometimes they randomly pop up, the images of him and another woman, while I'm just out and about enjoying part of my day and then it's ruined. Why can't I shake these feelings? Why does it bother me so much?? Whenever I genuinely express these feelings, most people tell me to "grow up" or "mature", but I don't how to just get over these feelings like that. Please don't insult me, please give real advice, because I'm trying to get over it, I really really am! To preface this, I grew up Catholic and believing my husband and I would save ourselves until marriage. Then I stopped being Catholic and thought "oh sex before marriage is ok!"-- and maybe it is, but I'm 100% regretting not waiting now, and not waiting for a man who has waited for me. This pain is not worth it to me!
To anyone who has had these same emotions and gotten over them-- how did you do it?? Please share how you got over it, because I absolutely want to so so bad.
(To anyone who has not felt these feelings, please refrain from telling me I'm immature or a terrible person, because I know it's not that...)
Thank you