My flesh and my heart may fail...

    My flesh and my heart may fail,
        but God is the strength of my heart and my portion forever.
(Psalm 73:26 ESV)

 

This verse offers no end of comfort and many of us come to it often. Concerning this passage John Calvin rightly states

I am rather disposed to think that there is here a contrast between the failing which David felt in himself and the strength with which he was divinely supplied; as if he had said, Separated from God I am nothing, and all that I attempt to do ends in nothing; but when I come to him, I find an abundant supply of strength. It is highly necessary for us to consider what we are without God; for no man will cast himself wholly upon God, but he who feels himself in a fainting condition, and who despairs of the sufficiency of his own powers. We will seek nothing from God but what we are conscious of wanting in ourselves. Indeed, all men confess this, and the greater part think that all which is necessary is that God should aid our infirmities, or afford us succor when we have not the means of adequately relieving ourselves. But the confession of David is far more ample than this when he lays, so to speak, his own nothingness before God. He, therefore, very properly adds, that God is his portion.

John Calvin (2009-06-03). Commentary on Psalms - Volume 3 - Enhanced Version (Calvin's Commentaries) (Kindle Locations 2502-2509). Christian Classics Ethereal Library. Kindle Edition.