5 Another opinion indeed has been entertained, and another sense in these words....It has been evidently explained, my brethren, where God would have us to be humble, where lofty. Humble, in order to provide against pride; lofty, to take in wisdom. Feed upon milk, that you may be nourished; be nourished, so that you may grow; grow, so that you may eat bread. But when you have begun to eat bread, you will be weaned, that is, you will no longer have need of milk, but of solid food. This he seems to have meant: “If I had not lowly thoughts, but have lifted up my soul:” that is, if I was not an infant in mind, I was in wickedness. In this sense, he said before, “Lord, my heart was not lifted up, nor my eyes raised on high: I do not exercise myself in great matters, nor in wonderful things above me.” Behold, in wickedness I am an infant. But since I am not an infant in understanding, “If I had not lowly thoughts, but have lifted up my soul,” may that reward be mine which is given unto the infant that is weaned from his mother, that I may at length be able to eat bread.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)