We trust in people who let us down, or in ourselves, only to become weak. David's advice would be to believe what God has said and seek Him, rather than fight our own battles. Heaven is real, and heaven's joy and peace are for this life as well as in eternity. Don't move without God. Move with God or do not move.
Courage is to have heart.
Exercise your courage, even if you only have a little. Keep believing God, and He will add His courage to yours. He will encourage your heart. David was a victorious follower of God. He is a good model.