16 “My strength to You I will keep”. For those strong men have fallen for this reason; because their strength to You they have not kept: that is, they that upon me have risen up and rushed, on themselves have relied. But I “my strength to You will keep:” because if I withdraw, I fall; if I draw near, stronger I am made. For see, brethren, what there is in a human soul. It has not of itself light, has not of itself powers: but all that is fair in a soul, is virtue and wisdom: but it neither is wise for itself, nor strong for itself, nor itself is light to itself, nor itself is virtue to itself.
There is a certain origin and fountain of virtue, there is a certain root of wisdom, there is a certain, so to speak, if this also must be said, region of unchangeable truth: from this the soul withdrawing is made dark, drawing near is made light. “Draw near to Him, and be made light:” because by withdrawing you are made dark. Therefore, “my strength, I will keep to You:” not from You will I withdraw, not on myself will I rely. “My strength, to You I will keep: because, O God, my lifter up You are.”
For where was I, and where am I? Whence have You taken me up? What iniquities of mine have You remitted? Where was I lying? To what have I been raised up? I ought to have remembered these things: because in another Psalm is said, “For my father and my mother have forsaken me, but the Lord has taken me unto Him.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)