Yeah. Again, the more and more you reveal of the latest interactions with her, the more and more I fall back into that camp. I do think you would be better off to just move on without her.
But, I also definitely agree you should destroy that letter before temptation may have the chance to get the better of you. It was fine to write it as it can be very therapeutic to get your feelings out, even just on paper. But, to send it would only cause a lot of unnecessary drama, just as much for you as it would for everybody else involved.
If the two of them aren't going to work out, that will happen anyway. You don't need to be the catalyst to cause that reaction. Besides, I think you've been through enough with her. You don't deserve any more of her drama, and doing something like that would certainly bring you more drama.