The original Greek of Matthew's parable sheds light on the great gulf between the two debts Jesus discusses. The first servant owed the master "a huge amount" (Mt 18:24), a term which translates to over 100,000 years of wages. The second servant owed "a mere fraction" (Mt 18:28), amounting to about one-hundred days' wages in the Greek. The second debt could realistically be repaid. The first debt was impossible to repay.
It was ludicrous for the first servant to contend that he could actually pay the master back 100,000 years of wages (Mt 18:26). He was out of touch with reality. The realistic response would be to fall on his knees and beg for complete mercy (see Lk 18:13) rather than beg for the opportunity to repay his debt, which was impossible (see Ps 49:8-9). Yet this servant wanted to be justified through his own actions. Out of pity, the master forgave the debt. However, the first servant still hadn't changed his mindset or his heart. His approach was still to satisfy debts through payment, not a writeoff. So he arrested the one who owed him a debt (Mt 18:30).
If we attempt to justify ourselves with God by our own efforts, we will fail (Gal 5:4-5), we won't want to forgive others (Mt 18:30), and we condemn ourselves to be damned (Mt 6:12, 15; 18:34-35). Let us never lose sight of how much God has forgiven us — even for a moment. Then let us totally lose sight of what others have done to us. Let us live to forgive, and forgive so as to live.