From the early Church Fathers to now.
3 Patristic · 2 Catholic
“To thee, O God my God, I will give praise upon the harp: why art thou sad, O my soul? and why dost thou disquiet me?”
“I shall console myself and … not allow myself to be alarmed by my thoughts but to hope in you, my God, to whom I should also give thanks, for from you it is also possible to hope for salvation.”
“And again, in order that he may draw the sound from that sounding-board below, he addresses his soul: he says, "Why art thou sorrowful, O my soul, and why dost thou disquiet me?" I am in tribulations, in weariness, in mourning, "Why dost thou disquiet me, O my soul?" Who is the speaker, to whom is he speaking? That it is the soul to which he is speaking, everybody knows: for it is obvious: the appeal is addressed to it directly: "Why art thou sorrowful, O my soul, and why dost thou disquiet me?" The question is as to the speaker. It is not the flesh addressing the soul, surely, since the flesh cannot speak without the soul. For it is more appropriate for the soul to address the flesh, than for the flesh to address the soul. ...We perceive then that we have a certain part, in which is "the image of God;" viz. the mind and reason. It was that same mind that prayed for "God's Light" and "God's Truth." It is the same mind by which we apprehend right and wrong: it is by the same that we discern truth from falsehood. It is this same that we call "understanding;" which "understanding," indeed, is wanting to the brutes. And this "understanding" whoever neglects in himself, and holds it in less account than the other parts of his nature, and casts it off, just as if he had it not, is addressed in the Psalm, "Be ye not as the horse and the mule, which have no understanding." It is our "understanding" then that is addressing our soul. The latter is withered away from tribulations, worn out in anguish, made "sorrowful" in temptations, fainting in toils. The mind, catching a glimpse of Truth above, would fain rouse her spirits, and she says, "Why art thou sorrowful, O my soul?" ...”
“From this it is clear that both psalms have the same meaning. Those using them encourage themselves to have stronger hope, overcome the feeling of discouragement and await the salvation from God that will doubtless be given them.”
“"Why are you sorrowful?" etc. All that follows has been expounded above in the preceding Psalm.”
“My God. This word is singular; but the former “Elohim,” is plural, to intimate one God in three persons. (Worthington) — Harp. Hebrew cinnor, which Symmachus renders, “the psaltery.” The sons of Core were chiefly door-keepers: but they also played on musical instruments. (Calmet)”
The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.