From Bob Hostetler from Guideposts:
I pray “Lord Have Mercy” a lot. Many people do. Some pray it only in church, others say it only in desperate times. Still others repeat it often in the course of a day. I’m in the last category.
Jesus told the story of a tax collector and a Pharisee who went into the temple to pray. The Pharisee—an upstanding citizen and religious leader—preened and pontificated in his prayer, while the tax collector confessed his sinfulness and asked for forgiveness, saying, “God, have mercy on me, a sinner”—and thus “went home justified before God” (Luke 18:13, 14, NIV).