Wow, that is one way to handle the situation. If you have agreed to end it, then just move on for now. A letter professing your feelings might be sweet, but if you already made the decision then it is over. Time to focus on what is in front of you. Otherwise, it becomes an effort in hanging on to your relationship. All that being said, you can still communicate while you guys are at school. If you remain friends there are opportunities later at the Academy when she could come to visit you (Yearling Winter Weekend, 500th Night, Ring Weekend, and 100th Night specifically).
Good luck to you! Keep your eye on the prize.