You see the problem is that you havent done this right.
I must make clear that I mean that you havent done this right IN HIS EYES.
What you should have done was gone mad, chucked him out and then when it came to him actually leaving, and you didnt stop him, he threw in "Do you believe me" about going to his brothers, to plant a seed of doubt in your head. That was when you should have begged and pleaded with him to stay in order to stop him going to her, at which point he would have eventually agreed to stay after much begging on your part. Except you didnt. You were supposed to at least make him promise he was going to his brothers, and probably phone the brother to make sure he was there. Except you didnt. You were supposed to care where he was and what he was doing. And you didnt.
You were supposed to be more angry with OW than you are with him. You are supposed to blame her, and stalk her and harrass and abuse her so that he can justify what he did because you are a a nut job. Except you didnt.
From his point of view, there is no way he can come out of this with anything that can justify his behaviour, that will look even vaguely ok to other people or even with a woman fighting for him.
This is day one of him realising that he has fucked up everything and all for what? Nothing.
Be prepared for him to get nastier over the next few weeks. I am sorry to have to tell you this but better you be prepared for it. Even worse if she isnt interested in immediately moving on/in with him. He will need to blame you for ruining his life by finding out what he did, it wont be his fault for doing it in the first place.
Then the "I am so sorry, I dont know what happened, I might be having a breakdown" will start, this will be to get you to take him back. He will start to turn up to see the kids looking like he slept in a skip, not shaven (extra points if he has lost weight "I just cant eat") etc. When that doesnt work, he may well revert back to angry. All the while threatening to get full custody of the kids (if you really want to fuck with him, say yes ok you could do with a break!) , dont panic, they all do this. As if they would want, or could cope with, the kids full time. Its just a weapon that they think will make us break and bend to their will.
Sadly they all go through the same stages, so bloody predictable.
But, take comfort from the fact that, sadly, millions of women have been where you are now and a hell of a lot of us are on MN. So that means that we can assure you that you WILL get there. You will have bad days and good days and thats ok. You will get stronger and stronger. The fact that we have been there too means that we can help when he thinks he is being clever and you are scared. We can help when the emotional manipulation starts (for example......"How can you make me live apart from the baby, I wont bond with it!" you say "So desperate to bond you were shagging someone else when it was in SCBU...." and so on)
YOU ARE NOT ALONE 