How do you stop the guilt?

How do I stop feeling guilty for ruining the years I’ve spent on this person? How do I stop feeling guilty about the way I treated myself like trash? I should’ve walked away but both ex and AP convinced me it was not emotional cheating it was normal, I was overrreacting, ex manipulated me in 100 diff ways to make sure I stayed, while being horrible to me. Every time I brought up the cheating he would flinch and tell me relationships don’t work on resentment which I have. After a massively traumatic event, he couldn’t give me any support, He left me after I began to get difficult, started dating the AP (who was a close friend) almost immediately after. We’re in the same workspace/college, I have to look at them every single day, wondering what I did for people and mutual friends to shun me away. I’ve been rebuilding from scratch and it’s so so so hard. How did I let myself fall so low? I feel horrible that all this time he probably didn’t love me. I wanted to be loved and I loved. They think I’m the psychopath and I just wanted a fair relationship. I never treated anyone wrong but I cut ties because I’m tired. What did I do to deserve this and how do they get to stay happy throughout?