(((Logliyn2))) - Many hugs to you. I am in the same boat. It sucks. It is the one thing I cannot let go of.
On Nov 19, I posted a similiar thread asking if my WH was a special kind of evil since I was on to him and THEN he slept with her. The responses I got helped tremendously. First, that like you, many of us knew that they were heading down that path and warned them. Secondly, there was nothing we could do to stop them. They had a one-track mind.
One of the last responses was about WH realizing that he already betrayed me, so what is one more lie? That hit home. In our eyes, it is the HUGE step to go from a EA to a PA, but in his eyes he it wasn't that big, it was a logical step. He was already hiding and lying, doesn't matter if it is a few emails and texts or sleeping together. He had already made the decision to go behind my back.
Here's the thread, I hope it helps: