I lost my virginity to someone in a relationship

This all happened over a year ago but I still think about it. It started one day after class. He came and asked for my number and we started chatting. As a first date, we both wanted to take a walk around our uni campus and there were some spots we hadn't seen in the dark so we thought we'd just do it. After walking around for a bit, we sat down for a while and we were actually getting along quite well. We liked the same type of music and everything. We were getting very comfortable and very touchy.

While sitting, he told me he has something to tell me and then proceeded to utter the words "I have a girlfriend". I literally took my hands off him and just sat it complete silence. He asked how I feel about it and I said I don't know. I asked him why he approached me in the first place and he said that he thought I was cute and he wanted to get to know me. He didn't press me for anything more after that point, he just walked me to my accommodation and hugged me good night. I debriefed with a friend and she helped me see that this was actually very crazy of him to do.

So for reasons unknown to me, I chose to stay in contact with him. I was still flirting and having fun with him and I think I decided in that period that I would just have some fun with him. Looking back, it was very shitty of me but I found a way to justify it to myself. Mainly by using the "I'm not the one cheating" argument. Until I invited him into my accommodation one day and we were making out half naked. And then 2 nights later, we had sex and we would continue having sex for the next couple of months.

I eventually lost the ability to keep justifying it to myself and I got the ick from both of us and just ended it between us. He understood and we stayed friends afterwards. We still are. He transferred to another uni in his hometown (where his girlfriend lived too) at the end of the year. I don't know her or what she looks like or if they're still together but I can't help but feel like I betrayed her too as a woman.

I feel really shallow. I don't know what to do with the regret of doing something like this to another woman. Even a year later, it feels so heavy. I haven't been able to fir give myself for making those decisions.