Begin by reading and meditating on Colossians 3:12-13.
You need to forgive. You need to. …You need to forgive others because God in Jesus Christ has forgiven you. Your infinitely holy and just Maker has not held your sins against you, but instead has held them against his Son on the cross. Your faith rests squarely on this act of substitution.
Your refusal to forgive one who has sinned against you is a manifestation of hypocrisy – a telltale sign that either you have not experienced God’s forgiving grace, or that you take such grace for granted. Why do you withhold what has been given so freely to you? Have your offenders done worse than you? Are their crimes against you more severe than your crimes against God and others? When you refuse to forgive, it can only mean that you have not yet come to understand forgiveness, or you have been taking it for granted and have not sufficiently learned from it.
You need to forgive to make much of Jesus and his gospel. This is the real reason to extend forgiveness to the undeserving. …Forgiveness points us all back to our greatest need – reconciliation to God by way of his own work.
…While forgiveness is never easy, it is also never impossible – not for those who have been saved by the grace of God. For the grace of salvation not only secures your forgiveness and models it for you, but it also empowers forgiveness by giving you a new heart and spirit.
You can forgive because you learned it in the gospel. …The gospel compels you to forgive, and it enables you to do so.
Give yourself to meditation on the cross; learn forgiveness, and walk in it.
Taken from Chap.20 “Forgive” (found in Part Two, “The Gospel and Others”) in Note to Self: The Discipline of Preaching to Yourself by Joe Thorn (Crossway, 2011), pp.77-78.