KGirl, that's a painful journey you've been on for sure. When I was reading your summary I was thinking "this guy is seeing someone else" and sure enough, you discovered the same thing. Your H is involved in an emotional affair, which is to say that his feelings for his co-worker have gone over the line and he's feeling "in love" with her and therefore not with you.
As you've probably read, this is always initially based on fantasy -- when people feel "in love" they put the object of their desire on a pedestal, don't see any negatives, and in fact will twist negatives into positives. I mean, the woman is pregnant with her third child, that should be a "don't mess with this person's marriage" flag to any rational person but your H is able to just pretend everything is okay.
So what can you do? As AnotherStander has pointed out, read the 37 rules and start getting really good at "act as if". A couple things to think about:
Your H's decision to wander usually arose from some level of resentment he was carrying around. Once he found himself over the line, the easiest thing to do is to exaggerate the resentment, and invent new things to be resentful about in order to justify the affair he is having. That's why they say "believe nothing they say" because a lot of it is just nonsense used to justify the affair.
That said, you want to avoid doing anything yourself that will make him resent you more. Blaming him, laying on guilt, publicizing his affair, all that stuff will make him resent you more and drive you apart, as will pursuing him.
He wants space from you right now, so if you fail to respect that and pursue him two things will happen (1) he will run further away, and (2) your value will diminish in his eyes because you are putting yourself in a "one down" position.
That is why "get a life" and "act as if" are so very important. They re-establish your value both to him and to yourself. You want to be the prize to be won.
The third tenant is "180" which is to say look at the things that bothered your spouse over the course of your relationship, determine which ones you want to change *for yourself* and then change them. If you're doing it to put on a show for him or as a device to get him back, he will see through it. 180 has to be for you, for real, and a long term commitment -- something you do when no one else is watching or acknowledging.
Your best course of action at this point is to be friendly and polite, but also to have your own life, which is to say go out and have fun!
Acc
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015