Yeah, it's normal to try to think up alternate scenarios and woulda-coulda-shouldas but it's just the aftermath of a breakup. People leave for all kinds of reasons. This was yours and it is a good reason.
You did the right thing.
I survived my ex sleeping with other women, getting in a new relationship and you will too. I was actually glad someone else had to deal with his ish instead of me. He won't magically be better for the new woman. It wasn't nearly as painful as I built it up to be in my head. It wasn't even painful! Better her than me, I say.