It is hard to keep expectations down when they have a minor breakthrough. Keep doing what you were doing. You are doing great at not getting too serious in your talks with him.
I am really surprised he said that about leaving OW. Sit back and wait. He is thinking. Let OW do the damage. He will definitely be thinking about you while she is there. She is a mess...my guess is that he will realize he doesn't want any part of that.
The timing on the letter was great. I think that is very important. If you had written it sooner I don't think it would have had the outcome it did. You gave him time to experience life without you. You let him realize that no one else can make him happy...HE has to do that. Good job!!!
Now is when patience is the most crucial....and also the hardest! I know you can do it though.
