This is one place where Catholic theology differs from most Protestant theologies. We all agree that through Christ's sacrifice we have forgiveness of sins, but Catholics recognize that Christ has covered the eternal payment due for our sinful nature, specifically, our spiritual death, our eternal separation from God. Jesus never promised us that his sacrifice would fix the temporal problems we have created for others when we sinned. In fact, Jesus tells us that it is up to us to deal with the temporal situation:
Therefore, if you bring your gift to the altar, and there recall that your brother has anything against you, leave your gift there at the altar, go first and be reconciled with your brother, and then come and offer your gift. (Matthew 5:23-24)
Here we see Jesus telling us (through the apostles) to make amends with the person whom we have wronged (or who has wronged us). Clearly it is unreasonable to expect that we will be able to undo every evil that our sins have caused, especially as we consider sins of months and years past -- who can even remember all the sins they have committed? We pray that God recognizes our efforts to correct those we do remember and can correct. But this does not mean we can just "write off" all the evil as unfixable just because some of it is beyond our power to correct! The person I insulted yesterday might still be hurting, even though I have asked God to forgive me for my unkindness (and therefore I am not "hurting"). What reason could I possibly have for not going back to that person and righting the wrong I have committed by apologizing (at the very least)?!
If we hold ourselves unaccountable for the ramifications of our sins, what is stopping us from living in a manner truly detrimental to our neighbors, and excusing our actions as being "covered" by the grace of God through belief in Jesus Christ? Imagine how terrible this life would be if we submitted to injustice in the flesh at the same time as we benefited from justice in the spirit! I will not accept such hypocrisy from myself. Jesus demands that we "go and do likewise" (Luke 10:37), and I will accept that command with all it entails.