blah blah blah. Sad but true, it is part of the "cheater/MLC script". So many of the same things have been said to so many of the good people on this board. Spouses/Partners rewrite history. Yup.. mine has flucuated between being in a bad marriage for a year, to being bad for four years, to being bad for almost our entire marriage. I have been a good person, I have been called a B!t@h. I have had the ow tell me to my face how "sorry she is" and that "they never meant for it to happen". THese op are not great people. They are liars. Unfortunately, your partner will have to figure it out for herself that a relationship built on lies is doomed from the start.
Affairs are a rush of emotions, a high.. and that, is what she is living in right now. Sadly, right now she won't listen to a response you from regarding her letter. She is in a "fog". It's like she is high on drugs and no reasoning from you is going to help.
So, until she figures out what she is giving up, what are YOU going to do for yourself? Read the books, make your plan, talk to a DB coach. Good luck.. This sucks for everyone but we do know how you feel.