Me and my girlfriend broke up last year, we got back together recently but I found out she slept with someone. how should I cope/move past it?

I want to start by saying that I believe she had the right to do whatever she wanted when we were broken up, but SHE had set boundaries to which I agreed with. One of those boundaries were that she did not want to be intimate with me in any way if I was doing anything with someone else, this could be as little as flirting with another woman. I agreed to this as I wouldn’t want her to do the same with me. The months go on, in which we have sex several times, I’d sleep at her place, we’d reminisce about the past, etc etc. 6 months into the breakup, I eventually find out that she had sex with someone else about 3 months prior, and she begins to act completely careless about it, responding bluntly to everything. I remind her about the boundary that she herself placed, but she would keep saying we were not together, and she owed me nothing. Whenever I tried asking her how she would have felt if I did the same to her, she deflects, and comes up with a multitude of excuses, and once she runs out, she just returns to not caring and telling me to suck it up. We eventually get back together, I found out more about the circumstances around it, she deeply regretted it, and argued with the guy afterwards because of how guilty she felt or whatever. I found this out by reading texts between her and her friend, so I don’t believe it was a lie. While this does make me feel a little better, I just can’t seem to move past it. Whenever we have sex I think about it, whenever I see sex in media I think about it, everything just reminds me of it. Her and I do not believe in hooking up, we have only really been together sexually with each other. Am I wrong or immature for feeling this way? Of course I wouldn’t like it if the person I loved was intimate with someone else, but if there was no boundary set it’d be completely different. Why place a boundary if you’re going to not follow it? I shot down ANY interactions from women because she would bring it up. Why was that not done for me?

Me and my girlfriend broke up last year, we got back together recently but I found out she slept with someone. how should I cope/move past it?
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