Your situation sounds exactly like mine. At least what you've described does. My ex had an inappropriate relationship with a work friend too. Like yours I don't think she had a PA, but it was cheating nonetheless. She is now openly dating him, and even took him and my girls to Disney with her family. Those were great pictures to look at on Facebook.
I don't think you were a fool, but he definitely was. I'm sure you did everything you could, but when spouses get to the point that ours did there is nothing you can do or say to change their course. All they think about is how miserable they are with you, and how this new person makes them feel so good. They don't think about how temporary that good feeling is until the honeymoon is over, so to speak.
