Ephesians 4:26-27 says "In your anger do not sin: do not let the sun go down while you are still angry, and do not give the devil a foothold." You see, our anger against others isn't necessarily a sin, but it can quickly turn into one. Anger against others is a great way to give Satan a foothold. Some of us have let far too many suns go down on our anger.
As Christians we are called not only to forgive in the worldly sense, but to completely forgive and to try to forget as well. There still may be consequences because of someone's actions against you, but forgive them as if it never happened. God shows us "forgiveness and forgetfulness" not only for our own good, but also as an example of the forgiveness we should extend to others.
Wounds are made by others on us daily. And wounds take time to heal. Holding a grudge just irritates the wound, and if we continue to hold a grudge, our wounds will never heal. It is then that it is time to call on the Great Physician to heal your wounds. Forgive the other person, and you'll both feel better.
I encourage you, if Satan has any "footholds" in your life today, get rid of them. First, ask God to forgive you. Then, forgive the other person. Finally, ask the other person's forgiveness for holding it against them. Love that person with God's love.
Forgiveness should be just one of the things that others see is different in Christians. Being able to forgive takes a lot of work, but if others see you can forgive, perhaps you will have a chance to lead them to Christ.