By not forgiving, you are slowly eating yourself up. Your mind works overtime unnecessarily. Proverbs 14:30 A peaceful heart leads to a healthy body; jealousy is like cancer in the bones.
A healthy body starts with a positive thinking. Un-forgiveness is a cousin of pride. To forgive, starts with understanding that when you forgive, you have nothing to loose and more to gain. When you are unable to forgive, it means your pride, talks to you more you can hear any other thing.