<human>: Can you summarize the given text in a few sentences?

My girlfriend is great. She's 19, I'm 22, we've been dating for about a year now. We get along wonderfully and live together, and very rarely have any disagreements. We have known and been into each other since high school (6-7 years ago), but never made the move until before my final year of college when I moved back to my home town. 

In that time, she had a few sexual partners, as did I. I learned about these through natural conversation, and each new realization was like a bullet in the chest. But I usually got over it and realized "well, who cares, she's with me now".

But the feelings keep coming back, and they're causing serious jealousy issues for me. Just this morning, we were going to "be intimate", and for no apparent reason I had a vivid mental image of her with one of these previous guys. I couldn't get back into it and had to stop and tell her I just realized I was running late for work. 

This only seems to happen every now and then, but it gets worse every time and all this morning I've had a twisted stomach and felt like crying. I just can't seem to figure out how to make myself okay with the fact that she's mine now (and our relationship is otherwise great), and that those previous relationships don't matter. She even cut off contact (for the most part) with the guys that weren't still genuine friends. This was just for my sake because she knew I was having issues, and without me even asking.

So, what can I do? Bringing up the topic feels a bit out of the question at this point. We've discussed it before, I've apologized and told her I feel bad that these irrational thoughts are causing trouble for us. At this point, I feel like it's an internal battle. I just wish I could be like all of my friends and just *not care*.
<bot>: Girlfriend has had several previous sexual partners, and I get vivid images sometimes when we have sex. Totally ruins it, of course, and I don't know what to do to make this go away.