There are two mantras that helped me when I was in a similar situation:
1) IT DOESN'T MATTER 'WHY'. You can ask this question all day long, you can get "answers" all day long, too. But in the end none of them will make you FEEL better. ACCEPTANCE is the thing that will heal your soul and heart most quickly. It is what it is, now what? How will I take care of ME? (Start with hiring a **** good lawyer.)
2) KARMA IS A BITCH. When your anger is seething and the bitterness kicks in (and it will) hand it over to God and trust that All Is Well. These affairs rarely work out and I'd bet my bottom dollar that, in time, he'll be miserable. Hopefully by that time you will have re-built your life, healed your heart, and have recognized long ago that he did you a big freaking favor.
You WILL get through it....one step at a time. But please don't let this affair define you or cause you to harden your heart to your own detriment.
And for Pete's sake, stop talking to him about anything. (Come to a quick agreement about visitation with the kids and then communicate ONLY about the kids and ONLY by email.) Whatever he says will only hurt and confuse you more. Focus on therapy for you and your kids -- you all need it.
I'm so sorry.