I read her letter -- twice. And here's what I think:
I think she loves you.
I think she is sincere in the thoughts she is expressing.
I also, however, still think she has it precisely BACKWARDS. If it were me, I would think carefully about it -- as Greek advises -- and then I'd probably reply "I'm sorry, wife, but you still need to make a decision here to end all contact with OM. Call it '1-2-1-2' , '1-2-2-1' or whatever you want to call it, but I have no interest in staying in a marriage with someone who's basically telling me I'm their second choice.
End your affair, and we'll talk. I think you'll find me, at that point, ready and willing to address all issues -- including my own."
