Please, help me to figure out how to break the cycle of bad behaviour we have both sunk into, I am miserable with him now, and would be miserable without him, but we had something so good and so precious not so long ago, and I want to find a way back to that.
Hi OP, I think, as others have said, until you can really truly say you can forgive and to a certain extent forget, then these battles will win out.
The fundamental thing needed to make a marriage work is Trust.
Your husband made a commitment to you on your wedding day, and it is and was his responsibility to honour that. Not the other woman's. her responsibility was to her husband.
Yes he might have been manipulated but he could have walked away. He didn't. He chose this path, but doesn't have the strength of character to own up to that.
You seem to be the one trying to 'fix' this, but I wonder if it is your place to fix this. Yes you made a commitment, and yes you want to make things work because marriage shouldn't be entered into lightly, but at the end of the day, 7 out of your 11 months of marriage have been a lie. You would be well within your rights to end this based on that.
It seems that if he can do this to you so early on in your married life, it makes me question has he done this before? Or will he do it again?
You must not in any way accept responsibility for his actions. Remember he chose to do this. If you can accept that and then actually really and truly forgive that then the arguments will stop. Otherwise the pain and hurt from his actions will continue to bubble under the surface and every minor disagreement will turn into another hours long argument. Add to that the effect of sleepless night, recovering from the birth of your new baby and all the other things that come with that, it isn't easy, not even for couples who have no relationship issues. Your focus will be on the baby, you will not have time to keep him afloat and nor should you have to.
You say you are miserable with him but would be miserable without him. Personally I would rather be miserable without him.
Also, in you thread title, you describe ourself as a sad pregnant lady and him as a sad angry man. What has he got to be angry about ???