(they should have talked to you about how unhappy they were, and -- if that failed repeatedly -- they should have sought a separation or even a divorce before sleeping around)
Isn't this what my WW did though? She met OM in mid-August. They quickly grew close and developed what I guess is an EA (no evidence). She realized her M was dead to her for a long time (had been complaining) and left me in early September. Then she developed a R with him and announced it to me in November. Didn't she do the right thing?
No, she didn't. She should have tried maybe one more time to get your attention, and then -- if necessary -- she should have told you she wanted a divorce, BEFORE hooking up with another man.
The whole "emotional back-dating" thing is VERY common with wayward women, especially. "Well I considered us pretty much apart at that point, sooo . . . . "