When we don't forgive another we are still attached to them by an invisible cord of suffering. It is like drinking poison and expecting another to die. Forgiving doesn't mean that we condone harmful actions...ever. But it does mean that we acknowledge that this person was acting from his or her wounds. We forgive because we want to be free of the harm that the not forgiving is causing us.
The other person doesn't have to know that we're forgiving them. But we know that we want our freedom. Seeing them as a small, innocent child helps us do this hard work. We know we're worth it. We love ourselves enough to work to be free.