I think I slept with my best friend's husband
Why is it just you think you did it? How do you not know?
I'm consumed with guilt, but at the same time I enjoyed sleeping with him.
You should avoid being alone with your friend's spouse. You did it once -- you can't take that back. You could ruin two marriages -- yours and your friend's -- by pursuing this any further.
Wait! What if I told you this was a confession?
You need to realize it's wrong, and then tell her and hope they can work it out.