2 “Lord, my heart is not lifted up”. He has offered a sacrifice. Whence do we prove that he has offered a sacrifice? Because humility of heart is a sacrifice....If there is no sacrifice, there is no Priest. But if we have a High Priest in Heaven, who intercedes with the Father for us (for He has entered into the Holy of Holies, within the veil),...we are safe, for we have a Priest; let us offer our sacrifice there. Let us consider what sacrifice we ought to offer; for God is not pleased with burnt-offerings, as you have heard in the Psalm. But in that place he next shows what he offers: The sacrifice of God is a troubled spirit: a broken and a contrite heart, O God, shall Thou not despise.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)