I think this is a really thoughtful post.
You absolutely have to draw your own line in the sand wherever that may be. There's no crystal ball that tells us if we've done the right thing, but as the above post says, you can change your mind too.
My situation with my ExP was a little different, in that he went and met a woman off one of those awful no strings sex sites. It all came out in the end and after time and soul searching I forgave him and we got back together.
Earlier this month, I found out he'd been messaging her again. The content of the messages was completely innocuous, but that wasn't the point. She wasn't someone he knew or worked with, there was no need for any contact between them. his only connection to her was that they'd previously had sex on multiple occasions behind my back. So I had no choice but to end things, once I knew they'd been in contact. It's 2 weeks on now and I'm still pretty sad about it, but I know that I've done the right thing. But I don't regret giving him that second chance previously.
Although people will say once a cheater or that he didn't deserve a second chance, I was an idiot for doing so etc, you have to do what you think is right. Our relationship was really good, we were incredibly compatible. I look at the stuff people complain about all the time on here, we had none of those issues. So to me it was worth it, but I knew if there was any kind of repeat that would have to be it.