Many people seek redress when they feel they have been wronged. It is a natural thing to do, in order to try and get back into the "perfect" world we have created in our own mind. Of course, the damage has been done, a wrong word has been uttered, or a regrettable action has been taken. When the supposed wrong-doer has apologized, the relationship is supposed to return to normal. But of course it seldom does. Sometimes it lingers silently for years, while outward appearance belies inward feelings.
One of my favourite verses is Philippians 2:3-4: "Do nothing out of selfish ambition or vain conceit. Rather, in humility value others above yourselves, not looking to your own interests but each of you to the interests of the others." I believe that in order to truly forgive others, one has to practice putting others above himself.
Whenever I find myself getting angry with someone, I remind myself that the problem is my ego (vain conceit). Once I have decided to humble myself, total forgiveness then flows easily. It might be someone who just cut in front of me while driving, or a young person who just acted disprespectfully. Or it might be a good friend who just said something hurtful, or a stranger who did not return due courtesy. If we expect to receive grace from God, we also must practice being gracious to others.