It really may not have to do with an apology. It's up to you. An apology may never happen, or, if it does, make you feel more like forgiving. Look at it as a control issue-a choice. There is a clear self-indulgent aspect of not forgiving.
Some people choose to continue to live with disruptive emotions.
You may need to make others feel guilty. The opposite choice is to give yourself the luxury of giving up those needs, purging those emotions, and moving on with your life. You take action. This act enables you to free yourself from the event, and let go. Then, move on.