Psalm 42 and 43 vacillate between postures of hopelessness and hopefulness.
The recurring theme of "Why are you cast down, O my soul, and why are you disquieted within me?" strikes me as an expression of deep hopelessness. The Psalmist has been remembering his times leading the multitude and participating in worship in the house of the Lord. Yet something has happened to make these experiences nothing more than memories for him now. His current state is one of being oppressed by enemies and persecuted by the ungodly.
His sights, however, are set not on his past joys or his present troubles but on hopes of better times to come - when he will "go to the altar of God, to God my exceeding joy and praise you with the harp O God, my God" (vs. 4). The cure for his [our] hopelessness is hopefulness - and not just in anything, but in God!
"Hope in God; for I shall again praise him, my help and my God" (vs. 5).