My (22M) girlfriend (21F) hid a past relationship from me. I’m feeling deeply hurt

A year into our relationship, I brought up a recent video I had seen wherein the girl hid some things relating to her virginity and hook up life from her boyfriend and how he then decided to leave her and stuff. My girlfriend then suddenly becomes quiet and then I ask what’s wrong. She then tells me how she and a guy we both know (her close friend, an acquaintance to me) and her had a month and a half casual fling a few months before we were together. They were cooking together one day and suddenly he kissed her. Apparently she also had a thing for him and they did stuff (no sex) for a month and a half. They then decided that they do not want to be together. This really destroyed me. This is a guy we see regularly, I see him in the gym daily, who my girlfriend talks to regularly as a friend and has said talking to him makes her feel better and more. When I pressed as to why she didn’t tell me, she said it would’ve been awkward and she didn’t want to spoil what we had. She let me go through their chats and it sort of shocked me. A lot of I miss you texts, she had told him about not getting her periods, about a pregnancy scare we had and way more. She said she planned on telling me once we were done with college and far away from him. If this was some random guy I’d get it if she hadn’t told me but this was someone she was close to, who we see everyday. I feel like I had a right to know. I hate that she hid this from me without even giving me a small hint that something like this had happened between them even when I’ve talked about him. She still occasionally texts him and she says he is genuinely happy for the both of us but I don’t know what to feel. I feel deeply cheated on even if that wasn’t the case. I need advice on what to do going forward.