RedDog - I can relate to your feelings. I put myself in a very similar situation with my ex, only to be caught by his pregnant fiance. I walked out of the apartment with a smirk on my face but inside I felt awful, humiliated, ashamed, you name it! Did my sleeping with him change anything? Nope - they still got married, she still had the baby.
This was the beginning of my end. I was totally embarrassed and could not believe I had but myself in that situation. What was I thinking?
Well, I wasn't thinking. I was just reacting. So I get it, I understand where you are how you are feeling.
Tight hugs hon - you can change this, starting today, right now!