IMHO, it sounds like you have not been able to forgive this person and forget this incident in your heart. It may be therapeutic and healing for you to be able to write down all the feelings you went through at the time. Simply opening up and writing it down may be enough for you. If you still feel like you want to mail it after, I think you should. When we harbor hurt in our hearts against another, Hashem takes that into account when judging them. Sending him a letter may open the lines of communication and bring you to forgive him.
Disclaimer: I am NOT a therapist or a Rav. This is simply my humble opinion.