Holding on to your anger is like grasping a hot coal with the intent of throwing it at someone else but you are the one who gets burned. It helps me to try to change my thinking to something, anything more pleasant so I don’t waste my time giving free rent in my head to someone who has upset me and who I cannot do anything about. The quicker I get over it, the happier I am.
I have heard the "free rent in your head" quote applied to forgive and forget ~ by not forgiving and forgetting only harming self, not the other who caused injury. The people who are fond of giving the advice do not explain how to forgive or how to forget. Also, "Those who can not remember the past, are condemned to repeat it." ~ George Santayana
Forgiving and forgetting is why so many women stay in abusive relationships. Yet they never truly forget; by saying "I forgive you," when he apologizes (do real men ever apologize?) and says "I am sorry, I will never do it again, I do not know what got into me, I was drunk...", do they truly forgive? It is not as if someone owes one money, and you say "That is okay, forget about it, do not have to pay me back, debt is forgiven." There is a tangible sign of forgiveness in that example.
"You have not forgiven them in your heart," is another thing I have heard said to people. Just as useless to me without an instruction booklet explaining how to go about the act of forgiving.