20 “But work Thou with me, O Lord God”. Some have thought “mercifully” should be understood, some have actually added it; but the best copies have the words thus: “But work Thou with me, O Lord God, for Your Name's sake.” Whence a higher sense should not be passed over, supposing the Son to have thus addressed the Father, “Deal Thou with Me,” since the works of the Father and of the Son are the same. Where although we understand mercy—for these words follow, “for sweet is Your mercy,”— because he said not, “In me,” or, “over me;” or anything of this sort: but, “work Thou with Me;” we rightly understand that the Father and Son together work mercifully towards the vessels of mercy. “Work with me,” may also be understood to mean, help me.
We use this expression in our daily language, when we are speaking of anything which is in our favour; “It works with us.” For the Father aids the Son, as far as the Deity aids Man, on account of His having assumed the “form of a servant,” to which Man, God, and to which “Form of a servant,” the Lord too is Father. For in the “form of God,” the Son needs not aid, for He is equally all-powerful with the Father, on which account He also is the helper of men....And because when he had said, “Work Thou with me,” he added, “for Your Name's sake,” he has commended grace.
For without previous deserving works, human nature was raised to such a height, that the whole in one, the Word and Flesh, that is, God and Man, was styled the Only-begotten Son of God. And this was done that that which had been lost might be sought by Him who had created it, through that which had not been lost; whence the following words, “For Your mercy is sweet.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)