In this particular Psalm, the psalmist encourages us to not put our lives in the hands of "experts" who know nothing about true life. "Mere humans don't have what it takes; when they die, their projects die with them." Instead, he encourages, put your trust in God, who always does what he says.
From Calvin: "David does not restrict the happiness of believers to present sense, as if they were only happy when God openly and in outward acts appeared as their helper, but he places their happiness in this, that they are truly persuaded of its being entirely by the grace of God they stand."
As Keith Green said, "Nothing lasts, except the grace of God, by which I stand in Jesus. I know that I would surely fall away, except for grace...by which I stand."
I write those words, and then find that joy is escaping me as my daughter is having a meltdown because she wants donuts. Once again, hope is fleeting. Once again, joy fades. Honestly, can I truly say that my joy is not based on circumstances when this particular circumstance sucks the joy right out of me? And here I have to go try to lead people in worship now. How can I do that in this frame of mind? I have to fight the spiritual desctruction that is threatening to take over my soul right now.