3 And since patience is needful in order to endure, until the harvest, a certain distinction without separation, if we may so speak (for they are together with us, and therefore not yet separated; the tares however being still tares, and the grain still grain, and therefore they are already distinct); since then a kind of strength is needful, which must be implored of Him who bids us to be strong, and without whose making us strong, we should not be what He bids us to be; of Him who said, “He that endures unto the end shall be saved,” lest the soul's powers should be impaired in consequence of her ascribing any strength to herself, he subjoins immediately,
“For Thou, O God, art my strength: why have You cast me off, and why go I mourning, while the enemy harasses me?”. I go mourning: the enemy is harassing me with daily temptations: inspiring either some unlawful love, or some ungrounded cause of fear; and the soul that fights against both of them, though not taken prisoner by them, yet being in danger from them, is contracted with sorrow, and says unto God, “Why?”
Let her then ask of Him, and hear “Why?” For she is in the Psalm enquiring the cause of her dejection; saying, “Why have You cast me off? And why go I mourning?” Let her hear from Isaiah; let the lesson which has just been read, suggest itself to her. “The spirit shall go forth from me, and every breath have I made. For iniquity have I a little afflicted him; I hid my face from him, and he departed from me sorrowful in the ways of his heart.” Why then did you ask, “Why have You cast me off, and why go I mourning?” You have heard, it was “for iniquity.” “Iniquity” is the cause of your mourning; let “Righteousness” be the cause of your rejoicing! You would sin; and yet you would fain not suffer; so that it was too little for you to be yourself unrighteous, without also wishing Him to be unrighteous, in that you would fain not be punished by Him. Consider a speech of a better kind in another Psalm. “It is good for me that You have humbled me, that I might learn Your righteousnesses.” By being lifted up, I had learned my own iniquities; let me by being “humbled,” learn “Your righteousnesses.” “Why go I mourning, while the enemy harasses me?” You complain of the enemy. It is true he does harass you; but it was you who “gave place” to him. And even now there is a course open to you; choose the course of prudence; admit your King, shut the tyrant out.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)