King David wrote this prayer after committing the mortal sin of adultery with a woman named Bethsheba. You may repeat in his words sorrow at sin, in humble supplication to Our merciful Lord, Jesus Christ.
Transcribed faithfully from the Douay Rheims Bible. For brevity, verses 1-2 have been excluded.
Have mercy on me, O God, according to Thy great mercy. And according to the multitude of Thy tender mercies blot out my iniquity.
Wash me yet more from my iniquity, and cleanse me from my sin.
For I know my iniquity, and my sin is always before me.
To Thee only have I sinned, and have done evil before Thee: that Thou mayst be justified in Thy words and mayst overcome when Thou art judged.
For behold I was conceived in iniquities; and in sins did my mother conceive me.
For behold Thou hast loved truth: the uncertain and hidden things of Thy wisdom Thou hast made manifest to me.
Thou shalt sprinkle me with hyssop, and I shall be cleansed: Thou shalt wash me, and I shall be made whiter than snow.
To my hearing Thou shalt give joy and gladness: and the bones that have been humbled shall rejoice.
Turn away Thy Face from my sins, and blot out all my iniquities.
Create a clean heart in me, O God: and renew a right spirit within my bowels.
Cast me not away from Thy Face; and take not Thy Holy Spirit from me.
Restore unto me the joy of Thy salvation, and strengthen me with a perfect spirit.
I will teach the unjust Thy ways: and the wicked shall be converted to Thee.
Deliver me from blood, O God, Thou God of my salvation: and my tongue shall extol Thy justice.
O Lord, Thou wilt open my lips: and my mouth shall declare Thy praise.
For if Thou hadst desired sacrifice, I would indeed have given it: with burnt offerings Thou wilt not be delighted.
A sacrifice to God is an afflicted spirit: a contrite and humbled heart, O God, Thou wilt not despise.
Deal favorably, O Lord, in Thy good will with Sion; that the walls of Jerusalem may be built up.*
Then shalt Thou accept the sacrifice of justice, oblations and whole burnt offerings: then shall they lay calves upon Thy altar.
*This psalm is excerpted from the Old Testament. Since the Coming of Christ, the new Israel is the Holy Catholic Church, and the rebuilding of physical Jerusalem (as was proper prior to Our Lord's coming) has been replaced by the building of the heavenly Jerusalem, that is, the Kingdom of God, via the evangelization of the Holy Gospels of Our Lord, Jesus Christ, and the call for all nations to believe in His Sacred Name, be baptized, and converted to our august Catholic religion which is without blemish or error. This set of truths contradicts the teachings of the Second Vatican Council promulgated by antipopes, see the Papacy page, but is nevertheless absolutely correct and must be held if one is to be truly loyal to the Teachings of the Holy Magisterium founded by Our Lord and faithfully held all the way up to our esteemed Holy Father, Pope Pius XII, prior to his death in 1958.