1 This Psalm is a short one; it satisfies the mental cravings of the hearers, without imposing too severe a trial on the hunger of those fasting. Let our soul feed upon it; our soul, which he who sings in this Psalm, speaks of as “cast down;” cast down, I suppose, either in consequence of some fist, or rather in consequence of some hunger he was in. For fasting is a voluntary act; being an-hungered is an involuntary thing. That which is an-hungered, is the Church, is the Body of Christ: and that “Man” who is extended throughout the whole world, of which the Head is above, the limbs below: it is His voice which ought by this time to be perfectly known, and perfectly familiar, to us, in all the Psalms; now chanting joyously, now sorrowing; now rejoicing in hope, now sighing at its actual state, even as if it were our own. We need not then dwell long on pointing out to you, who is the speaker here: let each one of us be a member of Christ's Body; and he will be speaker here....
2. “Judge me, O Lord, and separate my cause from the ungodly nation”. I do not dread Your judgment, because I know Your mercy. “Judge me, O God,” he cries. Now, meanwhile, in this state of pilgrimage, Thou dost not yet separate my place, because I am to live together with the “tares” even to the time of the “harvest:” Thou dost not as yet separate my rain from theirs; my light from theirs: “separate my cause.” Let a difference be made between him who believes in You, and him who believes not in You. Our infirmity is the same; but our consciences not the same: our sufferings the same; but our longings not the same. “The desire of the ungodly shall perish,” but as to the desire of the righteous, we might well doubt, if He were not “sure” who promised. The object of our desires is He Himself, who promises: He will give us Himself, because He has already given Himself to us; He will give Himself in His immortality to us then immortal, even because He gave Himself in His mortality to us when mortal....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)