Act of Contrition is the prayer the Priest would ask the penitent to pray after the absolution of sins. Here is the Act of Contrition in English from St. Alphonsus Ligouri.
Act of Contrition
O my God,
because You are so good,
I am very sorry that I have sinned against You
and by the help of Your grace
I will not sin again.
— St. Alphonsus Ligouri
If you want to pray the Act of Contrition in Spanish, click here.