I agree with Mike. You have analyzed very deeply... and I think it has come to a point to where you have to make a decision. A bit of a finishing blow... putting yourself out there. Of course it makes you extremely vulnerable to hurt... but you've hurt so much already by not letting it out. Even if you get rejected, it becomes possible to get past it, or better yet, to learn from it. Something that has caused you this much emotional angst is sure to make you much stronger for the future ahead of you.
The letter: I think that chances are that she WON'T talk to you about it, regardless of how she feels. If she didn't like you, well... you've pretty much covered that part in your post. If she does like you, though... she may feel awkward bringing it up. And that's where I think that a letter is not the best choice. I think it is much better to have a talk about this in person or at least on the phone.
If a dream comes true... then is it still a dream?