Here are some powerful words on forgiveness from Graeme Goldsworthy’s in his book Prayer and the Knowledge of God: What the Whole Bible Teaches. He is commenting on the fifth petition of the Lord’s Prayer, “forgive us our debts, as we forgive our debtors.”
“To show mercy and forgiveness to those who offend us stems from the conviction that nothing that others can do to offend us can compare with our sin against heaven. This is even more vivid when the offending person is a Christian brother or sister. To withhold forgiveness from them when we know they are forgiven and accepted by God is an almost blasphemous placing of ourselves above God.” (p. 101).