First version
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 occasions of sin.
Second version
My God, I am sorry for my sins with all my heart. In choosing to do wrong and failing to do good, I have sinned against you whom I should love above all things. I firmly intend, with your help, to do penance, to sin no more, and to avoid whatever leads me to sin. Our Savior Jesus Christ suffered and died for us. In his name, my God, have mercy.
Third version
O my God, I am sorry for my sins because I have offended you. I know I should love you above all things. Help me to do penance, to do better, and to avoid anything that might lead me to sin. Amen.
Further reading
Scott Hahn explores confession and reconciliation through Scripture and Catholic tradition, offering context for this prayer of contrition.
Lord, Have Mercy: The Healing Power of Confession by Scott Hahn (hardcover) on Amazon (paid link)