4 “Let us prevent His face by confession”. Confession has a double meaning in Scripture. There is a confession of him who praises, there is that of him who groans. The confession of praise pertains to the honour of Him who is praised: the confession of groaning to the repentance of him who confesses. For men confess when they praise God: they confess when they accuse themselves; and the tongue has no more worthy use. Truly, I believe these to be the very vows, of which he speaks in another Psalm: “I will pay You my vows, which I distinguished with my lips.” Nothing is more elevated than that distinguishing, nothing is so necessary both to understand and to do.
How then do you distinguish the vows which you pay unto God? By praising Him, by accusing yourself; because it is His mercy, to forgive us our sins. For if He chose to deal with us after our deserts, He would find cause only to condemn. “O come,” he said therefore, that we may at last go back from our sins, and that He may not cast up with us our accounts for the past; but that as it were a new account may be commenced, all the bonds of our debts having been burnt....The more therefore you despaired of yourself on account of your iniquities, do thou confess your sins; for so much greater is the praise of Him who forgives, as is the fullness of the penitent's confession more abundant.
Let us not therefore imagine that we have receded from the song of praise, in understanding here that confession by which we acknowledge our transgressions: this is even a part of the song of praise; for when we confess our sins, we praise the glory of God.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)