"God, be merciful to me, a sinner!" Luke 18:13
I doubt if many of us say this prayer the tax collector prayed, and that Jesus used as an example of a good prayer. I like reading stories of saints, and it is likely that a good number of them pleaded with God for mercy when they realized they were sinners.
There's Saint Augustine who sired an illegitimate son with his long time mistress. He left her in the hopes of marrying an heiress. Saint Callixtus was a world class thief who became a Pope! Then there's Saint Olga who out of revenge buried people alive or burned them. Her grandson St. Vladimir was bloodthirsty and offered human sacrifices. Saint Mary of Egypt was a prostitute who seduced all the crew and pilgrims on a pilgrimage to the Holy Land. Blessed Bartolo Longo who started out as a satanic priest.
But once they realized the enormity of their sin, they all turned to God, asked for mercy and forgiveness and changed their lives. Sometimes it's harder for us, who are not murderers or prostitutes, to realize how sin alienates us from our Father. Sin is sin, no matter how big or small. A red spot in an immaculate wedding gown can be seen from afar and will bother the bride. Lord, have mercy, for I am a sinner!