salm 42:5, 11 & 43:5 are all 3 almost identical verses. Psalm 43:5, "Why are you cast down, 0 my soul? And why are you disquieted within me? Hope in God: for I shall yet praise Him, who is the health of my countenance..." By the way, notice that the countenance of God (Psalm 42:5) and the countenance of the psalmist were expected to be connected. God's countenance affects our countenance. The expression on His face determines the expression on our faces... or at least, it should.
In principle, it seems apparent that David knew factually that he was supposed to be happy; he had good reasons to rejoice in faith, despite his negative circumstances. Yet, he wasn't happy. He was weary. David, with all of his fine qualities and attributes had a "disquieted soul" - that is, he was anxious, uneasy; he lacked the peace which he knew God could give him. Of course, he didn't have the advantage of reading Philippians 4:6-7. However, David knew what he needed anyway.
Psalm 43:3-4 is the heart of this song, "O send out Your light and Your truth: let them lead me; let them bring me unto Your holy hill, and to Your tabernacles. Then will I go unto the altar of God, unto God my exceeding joy: yea, upon the harp will I praise thee..." The light of God's countenance was something that David treasured; he valued it (Psalm 4:6 & 89:15). He knew he needed God's truth. He knew he needed God... because God alone is our inexhaustible source of joy; He is our joy (Genesis 15:1), if we will come to Him (Matt. 11:28-30).