The funny thing is that my wife did some terrible things during our seperation, I mean bad. I forgave her for everything, I have not once thought about them and gotten angry.
Side comment: "forgiving someone" is not the same as as "not getting angry".
But anyways, that aside; It sounds to me like the following is an essential timeline:
1. a few years ago, you had an affair
2. your wife tried to forgive you, but struggled, and after a few years, separated, and had one of her own
3. She wanted to make herself feel better about what she was doing, so "encouraged" you to have an affair also (this is very common, for the person having an affair to encourage their spouse to go date too. helps with their guilt, apparently).
4. conveniently for you, your prior affair partner was available, so you went right back to her.
5. Your wife seems to think, "well, he's got someone, and I've got someone... everthing's ok"
6. you think "well, she's got someone, and isnt interested, and I've got someone, so things are ok".
so that's where things stand right now. Meanwhile, your son suffers a broken home.
Here's the killer irony:
You say that "your 'brain' tells you that you are doing the right thing".
If you want to speak about logic... logically speaking, both of you get along way better than you did before, and neither of you are married again yet... AND you still keep thinking about her. So, the logical thing for both of you to do is to have some integrity and work things out between you, for the sake of your son.
your girlfriend "being a great person, and good with your son", doesnt make it the right thing to do, for you to stick with her. That just makes it "the easy thing to do".
there's a lot of missing details from your situation. If you had started up front with, "and my wife is now still seeing another man", it would have put a different perspective on things".
I think you should also stop beating around the bush, and say all these "terrible things" that your wife did during your separation. That would give peole a better insight into whether she is being vindictive, or possibly having her own MLC, or...
you should also mention whether it's been the same guy that she left you for, and stuff like that
My current status: june 2006. Wife ran out and filed D. Finalized Jan 11, 2010, after 12.5 years M. 3 wonderful sons caught in the middle