This is a horrible thing that has happened to you and I am sorry you are here. You have landed in a good and safe place though.
It is going to seem impossible right now, but I am going to tell you to take care of yourself and with a new baby, you probably haven't done much of that.
Then I am telling you to take care of your babies. They are the ones that need you to be calm and sane.
Your WH (wandering husband) has hurt you to the core and has acted selfishly. You spoke to the woman involved and whatever she told you...there may possibly be more. Try to be prepared for that.
Having life changes makes some of these guys get selfish and losing the attention of a wife due to childbirth is high on the list of stressors.
It doesn't matter. You did nothing wrong and you must believe that. This is allllll on him. Take some time to gather your wits about you, as you will have questions. He will tell you a few things, but more than likely he will hide much, in an attempt to minimize this mess.
If you have not done so, read in the FAQ in the yellow box at the left and read everything in the Healing Library that you can absorb. This is going to be tough for a long time.
You don't have to do anything just now. Try not to upset yourself more than you can help it. I am so sorry he did this, but no matter what else happens, you will be fine. You have to be, because your babies depend on you.
Take a shower and eat something. Someone else with more experience will be along to help. In the meantime, continue to read and post. Others will be here to check on you.
Don't be scared, honey. Be strong.