Ok Rob,I'm totally on your page now, she's just texted me about a nice apartment she has seen, She's going to come back home and want to talk about it, I think ww3 is going to break out in this house tonight.......
It's very simple, text her back, tell her:
"Good luck with the apartment, I hope it's something you like and it's something you can afford on your own because you need to realize that I won't be paying for it and also because that's where YOU will be living, I don't plan on swapping homes with you anytime soon. If you need help packing your things I can help by bringing some boxes home from work along with some packing tape, etc."
You don't throw in crap about being hurt and betrayed, you don't saying anything about doing this to get back at her. You just don't want to live with someone who could cheat on you, as far as you're concerned, you have decided that the relationship is over and it's time for both of you to move on.
That's how you start to let her go. You don't wait for tonight, don't wait for WW3 or WW4, just do it now. Instead of building anxiety within yourself over this issue all day long anticipating and fearing confrontation, give her something to deal with all day.
Do NOT wait, do it now.
"What is best for my kids is best for me"
Persevere = happily being patient over a long period of time