Most Helpful Girl
I feel that a letter not only shows that you put a lot of thought into it, but it is also a really good way of expressing your feelings, and especially for saying things that are hard to say face to face, yet need to be said.
When my ex and I were having problems, I wrote him a lengthy letter of the issues I had and what needed to be done to fix them. I was there when I gave it to him and told him to read the entire thing. I think it's good to at least give it to her in person, but you can give her a little bit of time to herself to read it. Good luck, and I'm sorry you have to go through this! Breaking up is hard, especially when you're the one doing it.
Most Helpful Guy
You should never write a note to break up. If you can't tell the person face to face in person, you don't write a note.