Stop letting your WW blame you. Next time she blames you, strongly and directly say, "it was your choice to cheat".
Yes, I would also add "don't blame me for your horrible decisions" to that.
BB, your wife is one of the most disgusting waywards I've ever heard described here. A teacher having sex with a principal in a school full of children! That just absolutely turns my stomach. And she remains defiant in the face of getting caught, it just blows my mind. You can't treat this woman with kid gloves like you have been, that has got to stop. You are preventing her from hitting rock bottom and preventing yourself from detaching. Leave her be.
Me: 59 w/ S17, D23, D26 Current R: 4 years Previous M: 21 years; BD: 06-14-12; S: 09-10-12; D final: 03-17-14; XW:56