I think we are both in the exact same boat: I also did the "be friendly" thing, and then just couldn't do it anymore. It was morally and ethically wrong, and you have to draw a line somewhere.
I told W the same thing, and then went dark. She also started responding very positively.
What I think is this: they can't believe their absolute good fortune initially in that they get booty on one hand, and their spouse is just treating them like gold, and waiting on them like a puppy dog (sorry PDT if you read).
Then all of a sudden, the spouse/BF is gone, and what feelings they have get cut off, and it makes them miss us. Now, it may simply be like missing an old pair of shoes - not because of love at that point.
So, now it's just spouse/BF and the OM/OW. At first, it's grand and glorious. No other half to annoy them, and they can do what they want. But I insisted to my friends that the polish wears off eventually, and sure enough!
I believe this is what has happened to you. But the real kicker is two-fold: 1. Are they strong enough to overcome whatever weakness caused them to leave in the first place? 2. Are they willing to completely put away the OM/OW?
To me, #2 is exactly what your email needs to state in unequivocal terms, but #1 is a tough one. I've said before I wouldn't start a conversation with W until she agreed to that.
