As a coach I would tell you to reread your letter and think of why you are writing it and what you wish the outcome to be. Firstly as an adult are you writing the letter with derech eretz? Secondly, are you writing the letter to let off steam or to let him know how what he did to you changed your life for the bad and it can have an effect on other children? Thirdly, what do wish the outcome to be? Do you want him to respond to you, or to just understand how he hurt you and not to do it to others?
Once you write this letter and send it off, do you expect to feel productive and accomplished? Will it improve your self esteem because you feel you have given tochacha to someone who once hurt you and you might prevent him from harming others? Will you feel that you have closure on this issue and you can now move ahead with your life?
If writing and sending the letter will be a positive experience and one that will give you closure to move ahead then by all means go for it. If for any reason you will regret sending it because you wrote it with bad intentions or you were rude or as hurtful as he was then don’t do it. Hashem is the ultimate scorekeeper so you can let go of it and don’t have to pay him back. Hashem takes care of punishing everyone for ALL their misdeeds and their victims do not have to even the score.
Forgiving someone is the greatest gift one can give oneself. When we are angry at someone they don’t necessarily know it but what happens is we place ourselves in a prison of anger and resentment. We tense up when we see them or think of them. We constantly remind ourselves that we are angry at them and we repeat the experience in our minds and hearts. We victimize ourselves over and over again when we remind ourselves of the incident and we experience it over and over again while our abuser or victimizer goes about their lives not giving us a second thought. So who loses out and suffers when we hold a grudge and do not forgive? We do. So really, forgiving is a gift we give ourselves and it is a self healing accomplishment.