10 “My righteous help is from the Lord, who makes whole the upright in heart”. The offices of medicine are twofold, on the curing infirmity, the other the preserving health. According to the first it was said in the preceding Psalm, “Have mercy on me, O Lord, for I am weak;” according to the second it is said in this Psalm, “If there be iniquity in my hands, if I have repaid them that recompense me evil, may I therefore fall by my enemies empty.” For there the weak prays that he may be delivered, here one already whole that he may not change for the worse.
According to the one it is there said, “Make me whole for Your mercy's sake;” according to this other it is here said, “Judge me, O Lord, according to my righteousness.” For there he asks for a remedy to escape from disease; but here for protection from falling into disease. According to the former it is said, “Make me whole, O Lord, according to Your mercy:” according to the latter it is said, “My righteous help is from the Lord, who makes whole the upright in heart.” Both the one and the other makes men whole; but the former removes them from sickness into health, the latter preserves them in this health.
Therefore there the help is merciful, because the sinner has no desert, who as yet longs to be justified, “believing on Him who justifies the ungodly;” but here the help is righteous, because it is given to one already righteous. Let the sinner then who said, “I am weak,” say in the first place, “Make me whole, O Lord, for Your mercy's sake;” and here let the righteous man, who said, “If I have repaid them that recompense me evil,” say, “My righteous help is from the Lord, who makes whole the upright in heart.” For if he sets forth the medicine, by which we may be healed when weak, how much more that by which we may be kept in health. For if “while we were yet sinners, Christ died for us, how much more being now justified shall we be kept whole from wrath through Him.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)