We must learn to fight despondency. The fight is a fight of faith in future grace. It is fought by preaching truth to ourselves about God and his promised future.
This is what the psalmist does in Psalm 42. The psalmist preaches to his troubled soul. He scolds himself and argues with himself. And his main argument is future grace: “Hope in God! — Trust in what God will be for you in the future. A day of praise is coming. The presence of the Lord will be all the help you need. And he has promised to be with us forever.”
The battle against despondency is a battle to believe the promises of God. And that belief in God’s future grace comes by hearing the Word. And so preaching to ourselves is at the heart of the battle.
– from John Piper’s Future Grace