Most Helpful Girl
I think that writing the letter would be good for you but I wouldn't give it to her. Write it and then rip it up and try to gain closure over the situation for yourself. Unless you find out she still has feelings for you or something, you don't want to give her a letter confessing your feelings for her like that.
I know how you feel about thinking about her everyday. I still think about my ex daily and we've been broken up for a lot longer. I even see him occasionally but I'm able to compartmentalize my feelings and act cordial--friendly even--with him and he acts normal with me now. That takes time though and it took a while for us to get to that point. But we don't talk outside of in person interaction and that's only in large groups with mutual friends.