Let's find what I think is one of the all-time greatest Catholic prayers...
The Act of Contrition.
O my God, I am heartily sorry for having offended Thee, and I detest all my sins because of thy just punishments, but most of all because they offend Thee, my God, who art all good and deserving of all my love. I firmly resolve with the help of Thy grace to sin no more and to avoid the near occasion of sin. Amen.
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The Act of Contrition