4 Hear ye all His rewards. “Who forgives all your sin: who heals all your infirmities”. Behold His rewards. What, save punishment, was due unto the sinner? What was due to the blasphemer, but the hell of burning fire? He gave not these rewards: that you may not shudder with dread: and without love fear Him....But you are a sinner. Turn again, and receive these His rewards: He “forgives all your sin.”...Yet even after remission of sins the soul herself is shaken by certain passions; still is she amid the dangers of temptation, still is she pleased with certain suggestions; with some she is not pleased, and sometimes she consents unto some of those with which she is pleased: she is taken.
This is infirmity: but He “heals all your infirmities.” All your infirmities shall be healed: fear not. They are great, you will say: but the Physician is greater. No infirmity comes before the Almighty Physician as incurable: only suffer you yourself to be healed: repel not His hands; He knows how to deal with you. Be not only pleased when He cherishes you, but also bear with Him when He uses the knife: bear the pain of the remedy, reflecting on your future health.... You do not endure in uncertainty: He who promised you health, cannot be deceived.
The physician is often deceived: and promises health in the human body. Why is he deceived? Because he is not healing his own creature. God made your body, God made your soul. He knows how to restore what He has made, He knows how to fashion again what He has already fashioned: do thou only be patient beneath the Physician's hands: for He hates one who rejects His hands. This does not happen with the hands of a human physician....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)