SW,
Here are a few comments:
On the Letter
Your AP has asked for no contact. If you and your husband send a letter then it could be seen as harassment. Focusing on the AP is something that many betrayeds do after DDay. It becomes a point of focus because of the inner feelings which basically boil down to the question "Why was I not enough?"
There is no answer to that, because the betrayed is focusing on what they believe are inadequacies in the relationship, but the truth really boils down to this: It had nothing to do with if the BS was enough, or not... It boils down to we were so broken and screwed up that we grabbed on to the nearest willing body to try to sate what need to be lacking within us. Unfortunately nothing fills that emptiness.
You are still in sprint mode trying to get everything setup and taken care of so that you two heal and move on.
Quit focusing on your AP, because its not about him. You have to come to a deep understanding that your choice of AP wasn't anything special. If he had been blessed with strong boundaries, you would have simply moved on and found another warm body to use in order to fill that black hole of needs inside of you.
While this sound good on the surface, a NC letter doesn't mean shit if the wayward still wants to contact the AP. With all the tech available waywards can do shit loads of stealth in order to keep an affair going.
Personally I saw your need for a NC letter coming from you as a "Grand Gesture" to show your husband that you are putting on paper the rejection of the AP. Here is the rub after dday WORDs don't mean shit, its all in the actions.
No contact letters that take the AP to task for their actions are pretty much a mental vomit and just show that the person who is writing it is still mentally tangled up with the AP.
The best no contact letters are simple direct statements that include the following basic info:
I hurt many people doing what I did
I telling you that contact is being cut off
If you do contact I will show my spouse
Please respect my wishes.
No blame the AP, no fire and brimstone falling down on them, no goodbyes, no I'm sorry, just the fact all contact has ended and you are just formalizing it in writing.
On the comment
In the fallout after an affair comes to the surface everything you do, say, think comes under the microscope.
It goes with the whole mindfuck that we put on our BS when they wonder about the whole "Why am I not enough." You were thinking about why was the letter so important, as we pointed out the AP had already asked for NC but yet you were still lingering on the thoughts of putting this letter together to send out to somehow prove to your husband that its over.
Again, the statement you made about someone pining for you reeks of star crossed lovers doomed to never be together, or that the fates have made it that you have been trapped by circumstances.
Your BS opened up to you about that statement because you put it forth about the AP, who in reality was nothing than a handy penis, and the man who vowed to love you is wondering why those thoughts don't apply to him. It speaks to the deep hurt which he is going through.
Finally you won't be able to explain that feeling to him because its foreign to him. Its like trying to explain to my wife about putting up a 5th of rum to my lips and chugging 1/2 because I needed it inside me just to make me feel that I would be able to handle life.