Congrats on the letter!
Now that you've written it, remember that they don't give a hoot about you leaving. You're just one more number that leaves the 'flock'. In essence, they don't want you, because you don't want them.
I thought of sending in a letter and making it official; but why? For me, it came down to, with encouragement from others, that I was still under they're control and they only wanted me to write that letter.
In any event, good luck on your decision. Tons have DA'd officially here, and there's nothing wrong with it. It can be a great part of healing.