The ultimate cause of all spiritual depression is unbelief. For if it were not for unbelief even the devil could do nothing. It is because we listen to the devil instead of listening to God that we go down before him and fall before his attacks. That is why this psalmist keeps on saying to himself: “Hope thou in God for I shall yet praise Him…” He reminds himself of God. Why? Because he was depressed and had forgotten God, so that his faith and his unbelief in God and in God’s power, and in his relationship to God, were not what they ought to be. We can indeed sum it all up by saying that the final and ultimate cause is just sheer unbelief.
…The main trouble in this whole matter of spiritual depression in a sense is this, that we allow our self to talk to us instead of talking to our self. Am I just trying to be deliberately paradoxical? Far from it. This is the very essence of wisdom in this matter. Have you realized that most of your unhappiness in life is due to the fact that you are listening to yourself instead of talking to yourself? Take those thoughts that come to you the moment you wake up in the morning. You have not originated them, but they start talking to you, they bring back the problem of yesterday, etc. Somebody is talking. Who is talking to you? Your self is talking to you. Now this man’s treatment [Psalm 42] was this; instead of allowing this self to talk to him, he starts talking to himself, ‘Why art thou cast down, O my soul?’ he asks. His soul had been repressing him, crushing him. So he stands up and says: ‘Self, listen for a moment, I will speak to you’. Do you know what I mean? If you do not, you have but little experience.
The main art in the matter of spiritual living is to know how to handle yourself. You have to take yourself in hand, you have to address yourself, preach to yourself, question yourself. You must say to your soul: ‘Why art thou cast down’–what business have you to be disquieted? You must turn on yourself, upbraid yourself, condemn yourself, exhort yourself, and say to yourself: ‘Hope thou in God’–instead of muttering in this depressed, unhappy way. And then you must go on to remind yourself of God, Who God is, and what God is and what God has done, and what God has pledged Himself to do. Then having done that, end on this great note: defy yourself, and defy other people, and defy the devil and the whole world, and say with this man: ‘I shall yet priase Him for the help of His countenance, who is also the health of my countenance and my God’. – D. Martyn Lloyd-Jones, Spiritual Depression