To my mind, harboring (holding onto) bitterness is inconsistent with real forgiveness. Bitterness is not the exact same thing as unforgiveness; bitterness is the *result* of harboring unforgiveness. Forgiveness is giving up your lawful claim or 'debt' you are holding against someone. If you still harbor an attitude that says, They owe me, or I will make them pay!, then you have not genuinely forgiven them. Without the help of the Spirit of God in Christ, is impossible to give the kind of forgiveness taught and exemplified by Jesus, but with His almighty Spirit of love working in our hearts, it can (and simply must) be done.