As noted at the beginning of Psalm 42, many commentators consider this psalm to be a continuation of that psalm.
Psalms 43:1 ¶ Judge me, O God, and plead my cause against an ungodly nation: O deliver me from the deceitful and unjust man.
At this point the psalmist is asking for God to vindicate him before his enemies. He wants God to take action against his “ungodly” enemies—against “the deceitful and unjust man.” He speaks with the confidence that God will agree with his assessment of his enemies. That can only be true if he is familiar with what pleases God according to His word.
Psalms 43:2 For thou art the God of my strength: why dost thou cast me off? why go I mourning because of the oppression of the enemy?
The psalmist continues by proclaiming his faith while questioning why God has apparently abandoned him and allowed his enemy to prevail. I can so relate with his heart. I am reminded so often of a couple of scriptures…
Mark 9:24 “…Lord, I believe; help thou mine unbelief.”
Hebrews 11:6 “But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.”
Even as he questions God, he reminds himself that his only hope, his strength is in God.
Psalms 18:2 “The LORD is my rock, and my fortress, and my deliverer; my God, my strength, in whom I will trust; my buckler, and the horn of my salvation, and my high tower.”
Psalms 18:32 “It is God that girdeth me with strength, and maketh my way perfect.”
Psalms 59:17 “Unto thee, O my strength, will I sing: for God is my defence, and the God of my mercy.”
Psalms 62:7 “In God is my salvation and my glory: the rock of my strength, and my refuge, is in God.”
Psalms 43:3 O send out thy light and thy truth: let them lead me; let them bring me unto thy holy hill, and to thy tabernacles.
Psalms 43:4 Then will I go unto the altar of God, unto God my exceeding joy: yea, upon the harp will I praise thee, O God my God.
The psalmist knows that his only hope for getting back to the house of God is for God himself to light the way and lead him. When he gets back to the temple, he declares that he will express the joy of his deliverance in praise to God through his music.
Scripture is very clear in telling us that God and His word are the source of light and truth.
Psalms 25:5 “Lead me in thy truth, and teach me: for thou art the God of my salvation; on thee do I wait all the day.”
Psalms 91:4 “He shall cover thee with his feathers, and under his wings shalt thou trust: his truth shall be thy shield and buckler.”
Psalms 119:105 “Thy word is a lamp unto my feet, and a light unto my path.”
Psalms 119:130 “The entrance of thy words giveth light; it giveth understanding unto the simple.”
John 17:17 “Sanctify them through thy truth: thy word is truth.”
Psalms 43:5 Why art thou cast down, O my soul? and why art thou disquieted within me? hope in God: for I shall yet praise him, who is the health of my countenance, and my God.
This is the signature verse that connects this psalm with the previous one. For the third time (twice in Psalm 42 plus this verse), the psalmist chastises himself for getting discouraged and declares his faith in God to deliver him.