So sorry, love. I saw your post on Ashley's thread and so I'm glad you've made another one of your own. The other girl is, I suspect, hugely significant - in fact, what ever he says to the contrary, I suspect he wouldn't have left you or behaved like the shit he has if he wasn't forming an attachment to someone else.
It's a horribly common practice that people having an affair re-write history - it's the only way they can justify to themselves - and others - what is inexcusable behaviour. Consequently, although I think affairs are wrong, I've got some respect for people who are honest and say "actually, my marriage was pretty happy, or at least not unhappy, until OW/OM came along. Ok, it was a bit boring, we could have done with a bit of a shake-up, but that was my responsibility too." I think a lot of affairs happen in marriages like that, but of course people have to invent what they think are better reasons for their infidelity. Please note that I am excepting abusive relationships from this equation.
This is ghastly for you Seth, but if your suspicions about an OW are correct, in a strange way it might help you. You can then remind yourself that this is not your fault and that all these things he's saying are lies - to himself, to you and to others. Unfortunately, a lot of OW in this girl's position delude themselves that all he's saying is true, because how could they live with themselves getting involved with someone who has ditched his pregnant wife? Much easier for them to demonise the wife, because it gets them off the hook too.
The truth is - unless you have been abusing him, or he'd been telling you for years that he was desperately unhappy (in which case, why did he make a child twice?) there are no excuses for what he's done. Even if he had been desperately unhappy, the decent thing to have done was to stay with you when you are at your most vulnerable. What are his family saying about his behaviour - and his friends?
Hard as it is, try to adopt a Teflon shell to all this emotional abuse and keep telling yourself that you are the better person.