<human>: So basically, we broke up, I started seeing another girl. We fooled around a little but never went all the way, just because, I don't know why. I just wasn't feeling it.

Meanwhile my ex slept with two other men. One was just a one night stand. But the other was a reoccurring affair with a guy in a relationship. This guy is kind of notorious for sleeping around on his girlfriend, but she claims she didn't know that at the time. I showed up at a party one night and she just happened to be there. We talked for awhile, I knew what was going on but had kind of accepted it. It all kind of blew up when he showed up, with another girl, and tried to call my then-ex out for being upset and embarrass her. I instinctively stuck up for her, almost started a fight, and made him leave. Which led to a series of events that ended with us getting back together.

Now that we're together, I can't seem to get over the fact that she's slept with other people. I feel so immature. I find myself getting mad out of nowhere and bringing it up. I think about during sex with her. I think about it while I jack off. I think about it at work. Here's the kicker though, I think it might just be that he has a bigger dick than I do. No shit. I'm that low and immature. I had heard from a girl friend that had been with him that he was "hung" and I made a joke about it to my girlfriend while she was my ex. She told me that "yeah it's big but you're still better than he is at it." while we were broken up. So I don't think she had any reason to patronize me, as we weren't together and I wasn't bothered by it at the time. I just want to know how to get over this. I feel stupid.

Can you condense the text into three sentences or less?
<bot>: Girlfriend and I broke up, she slept with another guy. I can't get over it. I think just because his dick is bigger than mine. I feel very immature. How do I get over it?