Wow, I'm sorry your going through this... Must be overwhelming ... Need to know more of the story to be able to give sound advice.
Ditto... although I already suspect it's probably more a case of this:
'No, er, I didn't cheat on you, I only slept with her because she BLACKMAILED ME, yes, that's it, she blackmailed me, meaning I was powerless to say No as well as to fail to get and maintain an er*ction!'
What, like THAT kind of 'blackmail', you mean?
Otherwise, it's called, Call the Police and report the woman for harrassment or, in fact, RAPE! (- physical knifepoint, psychological knifepoint - what's the difference aside from atoms?). Unless what you're being blackmailed over is your own illegal activities, of course, in which case - welcome to one of your consequences, courtesy of Fate, that you *and* your wife *and* quite possibly all that sail in you, have to suffer!... unless you'd prefer to face your own music to ensure 'the nasty cow' gets forced to face hers?