I will pass the annulment questions on to our expert Fr. Jonathan, but in the very least, get a good attorney. I wouldn't make any agreements until you know the full legal implications of what you are doing.
Lastly, I don't know where you are in your faith but now is the time to turn to God and ask for guidance and strength for you and your children. He will not abandon you, for He loves you very much, even if this man doesn't.
Pray. If you know some formula prayers, use them, but if not, just open your heart to Him and He will listen. No one here can tell you if it is a good idea to fight for this marriage, but God can.
He can bring you peace when you seek clarity and peace will be with the decision to let go or resist; but let God lead you toward His Will and that's where peace is.
I will keep you in my prayers.