This might help you understand what it is you are doing when you forgive. We are in a dark room, which represents the disorder of unjust treatment toward you. As you stumble around for a match to light a candle, this effort of groping in the dark for a positive solution represents part of the struggle to forgive. As you now light the candle, the room is illumined by both the light and warmth of the candle. When you forgive, you offer warmth and light to the one who created the darkness.
You destroy the darkness in your forgiving.
Now here is what I am guessing you did not know about the light of forgiveness: That light does not just stay in that little room. It goes out from there to others and it even continues to give light across time. For example, if you shed light and warmth on people who have bad habits, they might be changed by your forgiveness and pass it along to others in the future.
Now consider this: If you give this warm candle of forgiveness to your children who give it to their children, then this one little candle’s light can continue across many generations, long after you are no longer here on earth.
I am guessing that you had not thought about forgiveness in quite this way before.