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.
Nota Bene: Any spontaneous and heartfelt prayer that tells God that you are truly sorry for all your sins, that you will mend your ways, and that you firmly intend to avoid what leads to sin is a good Act of Contrition.