I still say don't send it. I believe you feel this is "going all in". You're pushing your last chips into the center in a take it or leave it, last ditch effort to convince her to change her mind. I'm guessing she won't. Then what? You've basically undone all the progress you made over the past few months.
But that's just my opinion.
In your mind, if she doesn't change her mind after this letter, are you assuming that it means you're done for good and it's time to move to the next girl?