Hi Muddle, Just got caught up with your new thread. While I think your letter was very eloquently written and heartfelt I do not think that you should send it. For one is will piss your W off and two you want the A to end because your W wanted it to. If OM breaks it off with her because of the letter your W will resent you even more.
I also think that a separation talk might be a good idea. She needs to fully understand everything that she is about to lose. What does OM have to lose? Is he M? Does he have kids?