80 “O let my heart,” he says, “be unspotted in Your righteousnesses, that I be not ashamed”. He returns to the words of His body, that is, His holy people, and now prays that his heart may be made unspotted, that is, the heart of His members; “in the righteousnesses of God,” not in their own strength: for He has prayed for this, not presumed upon it. In the words he has added, “that I be not ashamed,” there is a resemblance to some of the earlier verses of this Psalm. Whereas there, in the words, “O that,” he signifies a wish, he has here expressed himself in the more open words of one praying: “O let my heart be sound:” so that in neither of these two sentences, each of which is one and the same, there is found the boldness of one who trusts in his own free will against grace.
While he says there, “so shall I not be confounded:” he says here, “that I be not ashamed.” The heart then of the members and the body of Christ is made unspotted, through the grace of God, by means of the very Head of that Body, that is, through Jesus Christ our Lord, by the “laver of regeneration,” wherein all our past sins have been blotted out; through the aid of the Spirit, whereby we lust against the flesh, that we be not overcome in our fight; through the efficacy of the Lord's Prayer, wherein we say, “Forgive us our trespasses.” Thus regeneration having been given to us, our conflict having been aided, prayer having been poured forth, our heart is made unspotted, so that we be not ashamed.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)