The Seven Penitential Psalms
Ps. 6 (O Lord, rebuke me not in thy indignation), Ps. 32 (Blessed are they whose iniquities are forgiven), Ps. 38 (Put me not to rebuke, O Lord, in thine anger), Ps. 51 (Have mercy upon me, O God, according to thy great mercy), Ps. 102 (O Lord, hear my prayer, and let my cry come unto thee), Ps. 130 (Out of the depths have I cried unto thee, O Lord), and Ps. 143 (Hear, O Lord, my prayer: give ear to my supplication in thy truth).
From the General Confession at Mass
ALMIGHTY God, Father of our Lord Jesus Christ, Maker of all things, Judge of all men: I acknowledge and bewail my manifold sins and wickedness, which I from time to time have most grievously committed by thought, word, and deed against thy divine Majesty, provoking most justly thy wrath and indignation against me. I do earnestly repent, and am heartily sorry for these my misdoings; the remembrance of them is grievous unto me, the burden is intolerable. Have mercy upon me, have mercy upon me, most merciful Father; for thy Son my Lord Jesus Christ's sake, forgive me all that is past, and grant that I may hereafter serve and please thee in newness of life, to the honor and glory of thy Name; through Jesus Christ my Lord. Amen.
From Saint Augustine
I have sinned, O Lord, against thee and in thy sight. My many offenses cry for judgment against me; yet, O Lord, thou hast promised that where sin hath abounded Grace shall yet more abound. And thy promise bids me not despair, for he who despairs of pardon denies thee to be a God of mercy. The multitude of my sins abaseth me to the dust, but the thought of thy Passion raiseth me; the nails and spear cry aloud that thou hast reconciled me by thy Blood. The soldier opened thy wounded Side, wherein, as in a cleft of the Rock of Ages, I hide myself from the foe and rest secure. Thou inclinest thy Head, O Crucified Savior, even in death as if to greet me; thou openest thine arms as if to embrace me. In that embrace am I willing to live, and in it do I long to die. Amen.
From Saint Jerome
Show me thy mercy, O Lord, for I am a sheep that is gone astray. O Good Shepherd, seek me out and bring me home again to thy fold. Deal favorably with me according to thy good pleasure, that I may dwell in thy house all the days of my life and praise thee forever and ever with them that are there. Amen.
The Sacrament of Penance
The Sacrament of Penance, also called 'Reconciliation' or simply 'Confession', is first and foremost a confession of sin to God. But it is done in the presence of a priest, acting as the representative of the whole Church, because sin not only affects us individually, but also the Body of Christ we're connected to. When we sin, we not only damage our relationship with God, but also with his Holy Church. That's why in the Sacrament of Confession we also gain healing and reconciliation with the Body of Christ of which we are a part. In addition to representing the Church to us, the priest also acts as a spiritual father to us, giving us counsel and 'penance' - that is, a prescription for healing through spiritual discipline what was injured through sin.
The penitent kneels and says:
Bless me father, for I have sinned.
The priest responds:
May the Lord by in your heart and upon your lips, that you may worthily confess all your sins; in the Name of the Father, and of the Son, and of the Holy Spirit. Amen.
The penitent confesses:
I confess to God Almighty, to Blessed Mary Ever-Virgin, to Blessed Michael the Archangel, to Blessed John the Baptist, to the holy Apostles Peter and Paul, to all the Saints, and to you father, that I have sinned very much, in thought, word, deed, and omission, by my own great fault. Since my last confession, which was [here tell the priest when it occurred] when I received absolution and performed my penance, I have committed these sins: [here confess specific sins]. For these and for all other sins which I cannot now remember, I am heartily sorry, I firmly intend to do better, and ask pardon of God; and of you father, penance, counsel, and absolution.
The priest may address some words of counsel to the penitent, and will then assign a suitable penance. The penance having been enjoined and accepted, the priest shall say:
Almighty God have mercy upon you, forgive you your sins, and bring you to everlasting life. Amen.
The Almighty and merciful Lord grant you pardon, absolution, and remission from your sins. Amen.