How can I (28NB) move on from the anger I feel towards my ex (28M) after he completely replaced me less than a month after our breakup?

My ex (28M) broke up with me (28NB) 3 years ago and I still can't find peace about the whole situation. We were together for 3 years, mostly during Covid, and I was very intensely in love with him. We were each others partners in crime and support system during a very difficult time. However, after the quarantine period ended, we both really wanted to meet new people and experience new things. So I floated the idea of trying an open relationship (In hindsight, a terrible idea).

He was very enthusiastic about it, and even told me that he had always felt like he was naturally a polyamorous person. I tried to put some rules in place so it didn't feel so overwhelming: we weren't allowed to sleep with anyone else, and we couldn't get with someone we had any romantic involvement in the past. Right away he broke the second rule, stating it was unfair, but I quickly realized it was because he had already someone in mind (for who knows how long).

So they started going out pretty much immediately. Actually on their first official date he slept over at her house (but promised me nothing happened). From day one I felt horrible about the whole situation. I'd call him crying and constantly expressed how hurt I felt about the whole situation, and how I wanted to go back to how things were, but he kept convincing me that I just had to find people to go out with and that I would feel better then. So I kept trying.

This lasted a whole year. It was, for numerous reasons, the worst year of my life. Things were getting really rough at home, the people I went out with weren't interested in being non-exclusive, and I kept rejecting them because I didn't want to hurt anyone. Meanwhile, he kept going out with this same person (and no one else), and I felt like I was losing my mind with jealousy and grief for what we were. But I didn't want to end things... I loved him too much.

It all came crashing down when one day a friend accidentally let me know that they were in love with each other, and that the girl he was going out with wanted to be his girlfriend (I think she used the words "I want to have what you have with them"). I called him as soon as I found out, crying and freaking out, and asked him if he loved her. He said yes.

After that everything is a blur, I tried talking to her, she said they would stop seeing each other, he called me crying (because of her), we talked, I cried, we kinda broke up, he went no contact for a while. I eventually accepted our fate.

But one day, a few months later, I found out that not only they were now boyfriend and girlfriend, but he had moved in with her almost immediately after we broke up. He was taking her to do all the things we used to do together, even very specific things that I thought were part of our story. I felt (and still feel) so betrayed. I feel angry, I feel replaceable, and I feel an overwhelming feeling of injustice.

How can he move on like nothing ever happened? He couldn't even bother to grieve for us, to give time to process everything. He just replaced me without a single thought. I keep wishing that they break up, that he finally feels an ounce of the pain he caused me. But they just keep moving on, happy and unbothered. How can I move on from this? How can I stop feeling anger and grief and having nightmares weekly about them? How can I find real closure?

TLDR; My ex replaced me without a single thought and I can't move on from the anger and the feeling of injustice 3 years later.