Here is an excerpt from a book by Philip Yancey called "What's So Amazing about Grace?". It has really hit home with me and is well worth a quick read.
First he quotes Henri Nouwen:
"I have often said, "I forgive you," but even as I said these words my heart remained angry or resentful. I still wanted to hear the story that tells me that I was right after all; I still wanted to hear apologies and excuses; I still wanted the satisfaction of receiving some praise in return - if only the praise of being so forgiving!
But God's forgiveness is unconditional; it comes from a heart that does not demand anything for itself, a heart that is completely empty of self-seeking. It is this divine forgiveness that I have to practice in my daily life. It calls me to keep stepping over all my arguments that say forgiveness is unwise, unhealthy, and impractical. It challenges me to step over all my needs for gratitude and compliments. Finally, it demands of me that I step over that wounded part of my heart that feels hurt and wronged and that wants to stay in control and put a few conditions between me and the one whom I am asked to forgive."
Yancey follows this up: "One day I discovered this admonition from the Apostle Paul tucked in among many other admonitions in Romans 12... "Do not take revenge, my friends, but leave room for God's wrath, for it is written: 'It is mine to avenge; I will repay,' says the Lord."
At last I understood: in the final analysis, forgiveness is an act of faith. By forgiving another, I am trusting that God is a better justice-maker than I am. By forgiving, I release my own right to get even and leave all issues of fairness for God to work out. I leave in God's hands the scales that must balance justice and mercy."