Devasted after loving boyfriend cheated. I need closure.

Lots of context, tldr at the bottom.

I (30F) have been with my now ex (27M) for a little over a year. We have been in the same friend group for over 4 years but were never close before we started dating.

I had been wanting to date this man since I’ve known him : I have always been attracted to him physically and to his personality. When finally ended up being single at the same time last year and started sleeping together / dating. It was a time in both our lives where we had time for each other as it was our last semester at university.

Everything was amazing. We had time for each other, our sex life was the best I’ve ever had by far, we had fun. Everything was moving quickly : he told me he loved me and wanted to be exclusive a little before a month together. We moved in together after around 6 months and were happy living together until recently.

During that time , he was starting his thesis working from home and I had a new job mostly remote as well. We would spend our lunch period together and most of our free time. We saw family a lot and friends too. I got overwhelmed by work because some people were leaving without getting replaced and I was more stressed. He got a part-time job. We weren’t spending as much time together but I mean we were still together a lot. I was happy and he looked happy too.

This went on pretty much the same until this September, when he went back to study on campus. I was taking a graduate course as well but working almost full time, so we really wouldn’t see each other as much. He started to get involved in student activities from the get go, think parties and such.

At the first event we met a new student (F21) “Sarah”. From the looks of it she seemed very interested in him, but I just thought it was cute that a younger student would be interested, I didn’t feel very threatened. I even joked about it to my boyfriend saying she wanted him. He didn’t react much.

So anyway he ended up cheating on me with her. I found out looking through his phone (we had an open phone agreement that none of us were using). He had some weird non explicit exchanges but when I saw she told him she loved him I understood that it couldn’t be innocent (if that matters he didn’t say it back, I’m not sure if it’s better or worse). He admitted to sleeping with her on two occasions. According to the messages they would also spend a lot of time at university together and he admitted to making out with her in his office (I find all of this super pervy).

I was shocked because I trusted him not to cheat, I thought maybe a small crush and then we’d talk about it. I still have trouble believing that he ruined our relationship for sex, though I think he might be more emotionally involved than he let on.

He was very apologetic, said he felt bad (but not enough to let me know instead of having to find out through messages). I wanted to stay because I love him so much but honestly I can’t do it and don’t want to stay anymore.

During the time I was trying to make it work I kept on asking why he would do it but he couldn’t say anything other than I’m sorry I hurt you, I wish I could take it back. He was sad I broke up, he says he really wanted to stay with me. I’m not sure what to understand from this.

I can’t expect closure from him, I’m not convinced that he has the maturity to admit it to himself, yet alone telling me why he did it. I don’t know how to move on. I feel like I need an answer but I will never get it. I’m looking for advice on how to move on, on why someone would cheat in a relationship that seems to go well.

TLDR; My boyfriend cheated on me, relationship felt almost perfect (to me at least). I need advice on how to move on and maybe explanations on why he would do this.