I’ve been praying this prayer every day during Lent. It’s called the Act of Confidence in Divine Mercy, written by St. Claude de la Colombiere:
Lord, behold a soul that is in the world so that Thou may exercise Thine admirable mercy to make it shine forth before heaven and earth.
Others glorify Thee by showing, through fidelity and constancy, the power of Thy grace and how sweet and generous Thou art to those who are faithful to Thee. As for me, I will glorify Thee by manifesting how good Thou art to sinners. In me Thou will show that Thy mercy is superior to all our malice, that nothing can exhaust it, and that no relapse, however shameful and culpable it may be, should make a sinner lose hope in Thy salvation.
My beloved redeemer, I have offended Thee gravely. But it would be worse still if I add to my offenses the horrible outrage of thinking that Thou art not so good as to forgive me.
In vain Thine enemy–who is also mine–sets new snares for me daily. He may cause me to cast away everything except the hope I have in Thy mercy. Even though I fall a hundred times, and my sins were a hundred times more horrible than what they are, I will always continue to hope in Thee.
This is from a little prayer book that is distributed by the Miles Christi priests of San Diego. This is the one I’ve been praying mostly, but I’ve also prayed others that I’ve found online, such as this one.