Antlers, don't beat yourself up. A lot easier for us to say these things from way out here, without the emotional hammer hitting us in the gut. I've completely blown it in the past too, and had angry arguments with my W I regretted immediately. Try to learn and move forward, what else can we do, right?
I say get a L, tell him/her everything, tell them what your position is, listen to their advice, and prepare yourself. These legal things really don't mean as much as you might think. Like I said, in my sitch, the closer we've gotten to legal papers, the more my W seems unsure, and the more she reaches out to me. Try to just go with the flow. Don't help her, but don't impede either.
You've done great so far Antlers. Amazing how just when we're feeling strong, our WAW can drop a bomb and we find out we weren't quite as strong as we thought. Remember, all the work you've done up til now proves to you that you're okay on your own. You need to meet this challenge knowing that you'll be okay no matter the outcome. That way you can think clearly and rationally.