From The Gospel of Matthew by Curtis Mitch and Edward Sri, reflecting on Matthew 18:21-35:
Jesus calls his disciples to a very high standard of mercy. We cannot speak words of forgiveness while harboring resentment. The Catechism reminds us that true forgiveness entails “a vital participation, coming from the depths of the heart, in the holiness and the mercy and the love of our God” (Catechism 2842). Admittedly, this is not always easy. Some injuries are so deep that it “is not in our power not to feel or to forget an offense” (Catechism 2843). Nevertheless, if we remember how much God has forgiven us then we can avoid becoming like that unforgiving servant who, though he was forgiven much, failed to forgive others.