No whacks upside the head from me JL. Sounds like you are doing much better. It also sounds like you put alot of thought into the letter that you wrote to him, applying DB where necessary. I hope that it gives him lots to think about before he makes any bad decisions.
I also think that it's good for you not to talk with him until you feel that you are ready emotionally. I think that you are doing great.
When you finally do talk to him remember--no begging or pleading--no blaming--and don't tell him how he feels. Validate and use I statements. Try to keep your emotions in check (very difficult thing to do).
And whatever you do, don't start attacking the OW. He will defend her to the death right now. Totally sucks but it's probably true. In fact, do your best not to focus on her.
Have a great 4th. I'm off in a couple of hours to the fireworks.