9 Take care that you do not begin to be in a worse case as creditor than as debtor, like the man in the Gospel, to whom his lord forgave all his debt, and who afterwards began to exact from his fellow-servant that which he himself had not paid, for which reason his master being angry, exacted from him, with the bitterest reproaches, that which he had before forgiven him. Let us, therefore, take heed lest this happen to us, that by not forgiving that which is due to ourselves, we should incur the payment of what has been forgiven us, for thus is it written in the words of the Lord Jesus: “So shall My Father, Which is in heaven, do also unto you, if you from your hearts forgive not every one his brother.” Let us, then, forgive few things to whom many have been forgiven, and understand that the more we forgive the more acceptable shall we be to God, for we are the more well pleasing to God, the more we have been forgiven.
Source: Letters (New Advent)