Well, first of all, without digging too deep into it, nice to hear an apology I would think.
My opinion for how you should respond would be simply, I got your letter, thank you for writing it. I appreciate the apology or something along those lines.
You asking where this came from - you might not like the answer. Best to let her tell you when she is ready, if you care to hear it.
M, you've come a long way. We all deal with our pain in different ways. Your way works for you.
I dont think you really have to make any kind of a decision about anything right now.
Your xw will let you know if she has anything further to say, regardless of whether you are in a relatinship or not.
Sometimes it is best to just let things unfold. You will know in your heart, without a doubt, what you should do.
I think your xw is going through her journey and this is a step on it. She is going to feel some terible lows coming to grips with what she has done. Still your job to let her walk it.