I've been following your situation from day 1. While I am impressed at your growth I have to throw a couple of thoughts your way. it may seem critical of me, but here goes.
Gabby was spot-on. I saw that letter purely an attempt to provoke a reaction. You tell yourself all the time that this is a purely unselfish gesture like the letter, like the sandwich, etc. I don't know your W or your exact circumstances, but to me those things seem like you are trying to show your wife how the "new crimson." The only reason you have a negative reaction, again the ONLY reason is not because you are trapped in a negative thinking cycle, but because you have attached something to your action. When you don't get the reaction you consciously or unconsciously want, your mind a has to find a reason for it and that usually is negative. That negativity can screw you up big time. (believe me on this)
If you have to write the letters. then write them. Put them in an envelope and put them a way.
You can show your wife how much you appreciate your son, by being a great dad. You said your W "knows you feelings" then why do you keep telling her them?
Also this whole business of going to church and telling yourself "you are going to church today for you, not to sit and wonder if W is going to be there." (I've seen you do this with other thoughts as well were W enters the thought) You've already lost. W is already part of the calculation even if it's to tell yourself you don't care if W is there.
I mean really, who else do you do that with? For example, I love my twin brother, he's as close to me as anyone. But I would never think, "I'm going to the family barbeque for me and not sit and wonder if my Bro is there." If he's there it would be awesome and I would like it. But ultimately, it doesn't matter to my day if he's there or not. You sound like you are trying to convince yourself that your W not being at church doesn't matter, but it matters enough to tell yourself it doesn't matter, if you follow me.
All this bring to this notion. You simply haven't detached. Detaching can be tough, but it is so necessary. You have to detach REGARDLESS of how things turn out to have a healthy relationship with your W or otherwise.
You simply can't hang your emotions or feelings on another. It's not fair to the other person and not good for you. I would suggest you look into that before you start making deposits because without detachment every deposit has strings.
I think detachment can be very confusing and it doesn't always mean what people think it means.
one thing that really helped me detach was looking at how my thought process played into things. Google "Cognitive distortions." and Cognitive distortions book." There is a book out there that really helps with this and can give you practical exercises to work on this. So you aren't just trying to do it on the fly.
Good luck.
That ^^^ is golden and I agree wholeheartedly.
Me:45 ExW:48 M:04/97 3 Bombs & 2 ReCons 1st BD 11/10 D Finalized 4/20 D-16 S-14 Going in one more round when you don't think you can. That's what makes all the difference in life.~Rocky Balboa