How about this: you've forgiven someone for X when your choices, actions, treatment of him is no longer about the pain X caused you. There are no thoughts of teaching him a lesson, being withholding, no worries about him thinking you were over it, no thoughts of revenge, no tit for tat, no resentment, no flinching from him as the source of the pain. The PAIN itself is no longer shaping your choices.
There might still be lingering sadness, lingering pain. But, the current and future R with the person is not about the pain. You might even begin to look at how experiencing the pain enriched your life in some unexpected way -- not to justify X, but to accept it and see how sometimes the worst things can help us move forward.
Of course, the fact that X happened may inform your future choices, but in a matter-of-fact kind of way.
In my case: Hey H, I'll plan to go to event Z on my own tonight. If you're here when I leave at 7, we can go together which would be great :-)
In your case: Hey H, I still feel insecure about your R with OW. I want us to work on rebuilding trust. I've been looking at how to do this and transparency is an important part of it.
