Cheating is wrong.
I want to put that first-because you may not know anything about my story.
BUT-so is trying to be someone you are not.
It is COMMON for people who are poly to struggle to fit into mono situations and fail miserably. FAIL often =cheat.
It's not RIGHT. It IS real.
I agree that he was SO wrong to cheat on you.
I can't help hearing the sentence where you said he agreed to TRY to be exclusive but had concerns that he couldn't-BECAUSE he had always had that issue.
That's sort of like saying, "I know you suck at baseball and you've never hit the ball in your life, but I want you to try to do it now because if you don't hit the ball we lose." Then getting pissed off because the person doesn't hit the ball. Most people would think/say "what were you thinking? Of course they didn't hit the ball."
Sex/relationships are obviously a MUCH bigger deal than baseball games. But the point is the same.
YES people do generally deserve a second chance.
But more than that, people deserve to be accepted for who they are.
It sounds very much like you knew in advance that PROBABILITY said this man wasn't capable of an exclusive/mono relationship. WHY doesn't matter. He may or may not be poly. But he sure as heck isn't exclusive/mono if he's never been able to manage it.
IF what you want/need is an exclusive/mono guy-you need to go find one of those, not try to change a guy who isn't into what you want/need.
I'm not trying to be a bitch. Seriously-I am just trying to show you HOW you can control whether or not you suffer from this situation AGAIN. We tend to repeat mistakes we don't learn from. Asking for exclusivity from someone who'd never managed it... that was a mistake...
He also made MANY mistakes here. But we can ONLY change OURSELVES. So there is no rhyme or reason in outlining his mistakes. Only he can change them. The only thing YOU can do is work on YOU.
"Love As Thou Wilt"