toomany - Of course, how were you to know? It's not what you planned, who does? But trust me when I tell you that it's not as bad as some people make it out to be. I was on my own with DD for 5 years before I met my DH.
We ended up in a hostel the day I left and stayed there for 3 months until we were housed. I never in a million years thought that I would be that person. Never. I'd been through it as a child and swore I'd do better. Thing is, sometimes, it's not within your control. You have to do what is best for yourself and for your children.
But I honestly believe that going through all that, surviving all that pain and upset has made me who I am today.
DD and I are so close. She knows why it was just us, she asks me about the times we were with him and she understands why we left. She's 12 now. I'm happily married and we have an adorable DS.
Life goes on sweets. It is what ever you make it. But YOU have to make it. You have to decide what is or is not acceptable behaviour.
And, from what I have read, this is far from what is acceptable for you and your children.
Take the steps towards a better life, however you do it, you need to start sooner rather than later.
Your life as you knew if when you married your husband has changed. I'm not sure if it could ever been the same again, even if you did stay together. It's going to be hard, which ever way you turn.
Lean on your friends and family, let it all out here. Talk to us, tell us how you are feeling. That alone will change hourly from wanting to make a go of it so 'she' doesn't win, to wanting him to burn in hell. Twice! 
Look after yourself. Look after your heart.