I wish I had a neat, simple answer for this question, but I don't. I guess because it is so hard, this makes it is such an important thing. If forgiveness was as easy as picking up a few groceries from the store, we wouldn't appreciate it as much. It wouldn't even make a dent on our emotional radar's.
Forgiveness is really life changing. The longer I refuse to forgive others the longer I live in bondage to that person - my unforgiveness gives that person/situation power to rule my life. It also prevents me from fully enjoying the forgiveness offered by God. My decision to hold onto the burden, grudge, and pain means that I haven't got an empty heart to receive God's blessing of forgiveness.
Eugene Peterson - "In prayer there is a connection between what God does and what you do. You can't get forgiveness from God, for instance, without also forgiving others. If you refuse to do your part, you cut yourself off from God's part. (Matthew 6: 14-15)
Living in Grace