I've (23M) known a girl (19F) for a long time (since we were kids) and up until last summer we were only friends; however, there was chemistry. So last summer, I came back from vacation outside the country and hit her up to hang out and we fucked. This is how we started dating casually. 2 months in, me, her, my cousin (24M), and two other friends go on a road trip (also no one knows we are dating but us). So a week before the trip she is taking pictures with my cousin, having her hand on him which I considered passive dread, but I ignored it (mistake?). We leave for the trip and two instances bothered me. Mind you, I've seen texts of her thirsting over my cousin while me and her were still friends so I know she is attracted to him to. The first one that concerns me was when we were all swimming and then she got out to check her phone. While I was chatting with the other people in our group, I noticed that she and my cousin were talking briefly and just left. I didn't think much of it at the time but they were gone for a good half an hour to god knows where. I talked to my cousin about it after the trip and he said they were at the gym but I haven't talked to her about it. In a way, I see her leaving off with one of my family members privately to get under my skin as disrespect (whether they fucked behind my back or not). Then they non-chalantly came back to the pool to chill before we all got out and went back to the Airbnb. The next day we go to an amusement park and I'm the only one who didn't pay online so I stand in line to get my ticket. While I'm there, these two decide to go back to the car (which was about a half a mile away) for almost an hour as the rest of the group waits at the front gate. They come back and we go in and have a good time. I'm usually the one to cut toxic people out of my life like a cancer without second thought but I thought she'll be back at college soon so I'll just keep my mouth shut and keep fucking her in this casual relationship. A few months go by, she cries and says she has feelings for me and wants more....I cave. Now I'm faced with slight regret because the memory of her disrespecting me while she's smiling and showing me off as her boyfriend. There are days IDGAF and then days when I say I made a mistake being in a serious relationship. She's a HS valedictorian going to an Ivy trying to be a doctor and I've seen some great mother-like qualities in her, I just don't know what to do about the past memory. Should I bring it up and seem like a bitch about what happened months ago? Should I keep ignoring it? Do you guys think it was dread or she might have actually fucked him? I just want to do something about this but I don't want to make more mistakes.