C. S. Lewis writes about the problem of forgiveness:
. . . you must make every effort to kill every taste of resentment in your own heart—every wish to humiliate or hurt him or to pay him out. The difference between this situation and the one in such you are asking God’s forgiveness is this. In our own case we accept excuses too easily; in other people’s we do not accept them easily enough.
As regards my own sin it is a safe bet (though not a certainty) that the excuses are not really so good as I think; as regards other men’s sins against me it is a safe bet (though not a certainty) that the excuses are better than I think. One must therefore begin by attending to everything which may show that the other man was not so much to blame as we thought.
But even if he is absolutely fully to blame we still have to forgive him; and even if ninety-nine percent of his apparent guilt can be explained away by really good excuses, the problem of forgiveness begins with the one percent guilt which is left over. To excuse what can really produce good excuses is not Christian character; it is only fairness. To be a Christian means to forgive the inexcusable, because God has forgiven the inexcusable in you.
This is hard. It is perhaps not so hard to forgive a single great injury. But to forgive the incessant provocations of daily life—to keep on forgiving the bossy mother-in-law, the bullying husband, the nagging wife, the selfish daughter, the deceitful son—how can we do it? Only, I think, by remembering where we stand, by meaning our words when we say in our prayers each night ‘forgive our trespasses as we forgive those that trespass against us.’ We are offered forgiveness on no other terms. To refuse it is to refuse God’s mercy for ourselves. There is no hint of exceptions and God means what He says.
C. S. Lewis, The Weight of Glory (New York: Harper Collins, 2001; Originally published 1949), 181-183 (paragraphing mine).