"I know that you have been in contact with OM. You have been lying to me and the MC. If this does not stop immediately and you don't agree to full transparency on keeping yourself accountable then I am going to file for divorce."
If she asks how do you know just say, "let's no kid ourselves, I have lots of proof."
This is about her behavior (contact with OM) it's not controlling because she has a choice. It's a boundary on how you will let her treat you.
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.
