When your mind is clogged up by grievances from the past, it blocks your happiness. You can let go but sometimes you just need to fix things. However long after the event it is, take a first step. Write a letter to the person in question, alive or dead, whether or not you have their address. Get it all out. Plumb the depths. Face the worst. Notice how you feel. Decide what to do. Send the letter or use it as a script for a conversation. Or even set light to it and scatter the ashes.