I am also fairly certain XABF cheated, but without actual proof.
I rid myself of these thoughts through the same slow, rigorous process I rid myself of all other torturous thoughts about stuff I couldn't know or control: through acceptance.
Acceptance that he was not who I wished he was.
Acceptance that I was in denial about the first one for a long time.
Acceptance that no matter what he did or didn't do, I could not change the past.
Acceptance that I never had to be in a relationship like that again.