SHORTER OPTION:
“Lord Jesus, son of God, have mercy on me, a sinner.”
OTHER COMMON OPTION:
“O my God, I am heartily sorry for having offended you, and I detest all my sins because I dread the loss of heaven and the pains of hell, but most of all because they offend you, my God, who are all-good and deserving of all my love. I firmly resolve, with the help of your grace, to confess my sins, to do penance, and to amend my life. Amen.”