23 Furthermore, just as Peter, after their having been scandalized by the hardness, as they thought, of the discourse of the Lord, even then said, “to whom shall we go?” so he has added, “Cast upon the Lord your care, and He shall Himself nourish you up”. A little one you are, not yet you understand the secret things of words: perchance from you the bread is hidden, and as yet with milk you must be fed: be not angry with the breasts: they will make you fit for the table, for which now little fitted you are.
Behold by the division of heretics many hard things have been softened: His discourses that were hard have been softened above oil, and they are themselves darts. They have armed men preaching the Gospel: and the very discourses are aimed at the breast of every one that hears, by men instant in season and out of season: by those discourses, by those words, as though by arrows, hearts of men unto the love of peace are smitten. Hard they were, and soft they have been made. Being softened they have not lost their virtue, but into darts have been converted....Upon the Lord cast yourself. Behold you will cast yourself upon the Lord, let no one put himself in the place of the Lord. “Cast upon the Lord your care.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)