Hi OP, i think i posted before but needed to post again, especially after reading this:
At the counselling thing he said he sometimes feels like I put work ahead of the family and he has felt a bit neglected since the pregnancy. I guess that is why having the undivided attention of the OW has come into it. I did work a lot more last year, in preparation for having the luxury of a slower second half to this year following the new arrival, and it was obviously something that was discussed, but it seems like this might have been the wrong approach in hindsight. Maybe I got a bit complacent about the relationship, we've been together forever and I didn't really think he still needed that assurance or attention of old. Not sure i'm explaining that very well.
FUCK THAT SHIT.
He made the distinct choice to cheat. It wasn't an accident. As some other posters have been saying to me, he didn't slip on a wet patch and fall on top of her naked or whatever the equivalent is.
I have made my own thread about reading the newish Chump Lady book - it is honestly amazing. I read the whole thing in one go yesterday.
It puts into words everything you have been saying here, and everything I have felt since D-Day. All the pressure he is putting on you, trying to make you feel guilty and unreasonable. Don't accept it. Determine what are choices you yourself are making and own them, and don't make choices for him/his sake.
As she says: "you weren't meeting my needs" should be met with "you weren't meeting my needs, either, and I didn't cheat on you. please don't try to pawn this crap off on me."
Also, she reminds constantly that people cheat because of something lacking in them - empathy and character - and that it is because they feel entitled to something.
He had choices if he was unhappy - he could try and work on things with you by letting you know and by trying counselling, or he could cheat, or he could just carry on or he could cheat. he chose the coward's way out.
I saw a new counsellor last week, who hit the nail on the head when she told me, "i hear a lot of blame. i hear you blaming and shaming yourself for this". You, and I, are NOT to blame for the cheater being a cheater - only he is.
Good luck, and don't forget to take care of yourself.