Yeah, it is a tough conversation. Be as civil and self-positive about it as you can.
Remember, all you are really doing is accepting her actions not to be in an R with you. By you saying, "I am not ok with staying married to someone who treats me and our M this way." or simply "I am not ok with this (anymore)," you are valuing yourself and, presumably, your core boundaries about who you want to be and how you want to be treated by others.
As much as you can, make it about what YOU are ok (or not ok with) in a relationship (general), not about what you wished SHE would have done or not done. And the boundary enforcement can even be almost apologetic, "Ya, I'm sorry, hon, but I'm not ok with this situation anymore. I have decided that it is best for ME to move on."
If she asks why or protests, "Well, I would have preferred to have worked things out with you, but I want to be in a R with someone who wants to be with me and who wants a committed R with me. I just don't feel that from you, and I know there are others out there that would feel that for me. I wish you all the best, W."
She is free to reverse her path over the course of the next few months if she changes her mind. Perhaps you will want her back if she does, perhaps, like Mikey from Swingers, not.
I will add to this with some advice that will ultimately help you feel better about yourself. I have not been able to get here in words, but in my head I am moving towards finding forgiveness for my wife. I am in a similar situation and I think the key for my long term happiness, at least part of it, is to let go of the resentment I feel towards my wife and ask her to forgive me for my contributions to the demise of my marriage. In doing so, I know that I will be at peace with myself and, if delivered with the utmost sincerity, will help your wife find some peace as well. Check out the 5 Languages of Apology by Gary Chapman - extremely helpful to me. If you can elevate to a higher ground, man, the world is your oyster...
