How badly did I screw up?! What SHOULD I have said?
A couple things Id recommend.
1) None of these things are urgent. If you need a few minutes or hours to think about your response, it's OK. If you need to post here first, then do it.
2) I would try to give 70% or so of what she says. Going on for paragraphs isnt particularly meaningful. Talking about your regrets or path forward with HER isnt particularly useful. Here's a phrase that my DB coach gave me that I think is wonderfully succinct and gets across everything you wanted to say very efficiently.
I do not believe that divorce (or separation) is the answer to our problems, but I love and respect you enough to move forward if thats what you choose. [Its been a couple years, so I may have a couple of the words wrong :)]
But I think that should be your focus. Listen to HER. Dont talk at her.