a. Depression is a chemical imbalance, and a physician may be
able to prescribe a correction.
b. Depression has psychological causes which a counselor may be able to disentangle.
c. Depression belongs to our normal human flesh, but the Holy Spirit can deal with that.
In the epistle to the Romans Paul has half a dozen very typical symptoms. "I do not understand my own actions. I do not do what I want, but I do the very thing I hate. I can will what is right, but I cannot do it. I do not do the good I want, but the evil I do not want is what I do. Wretched man (woman) that I am! Who will rescue me from this body of death?" (Romans 7:15-24).
Very roughly I will define my flesh as the natural instincts (drives) I received through the genes of my parents, and these were twisted by chemical and other traumas in my mother's womb, my early childhood experiences, and other disappointments and hurts in my life. Since my flesh was given to me by God, it cannot be sinful. John wrote that "the word became flesh" (John 1:14) which means that Jesus had fleshly feelings like all of us. The flesh only becomes sinful when I focus on it instead of the Holy Spirit.
As Paul explains, "those who are in the flesh cannot please God" (Romans 8:8). That is obvious because my natural instincts have no desire to be monogamous, turn the other cheek, go the extra mile, love neighbors, love enemies, bless unpleasant people, etc. (Matthew 5:27-47). Our instincts are just about identical to those of a cat (self-protection, territory, food, comfort, curiosity sex & mothering). And it is never natural for an animal, including the human animal, to love enemies, pray, forgo rights, etc. But then animals are not depressed by this. It is humans with high ideals who get depressed, and the higher our ideals and expectations the worse we feel. Unashamedly fleshly people, who let everything hang out, get along rather well.
Paul's solution for his depression (Martin Luther called it "the bondage of the will") was to avoid setting his mind on his flesh, and all its natural works. He did not expect to lose his twisted instincts, or eradicate them. Rather he bracketed the flesh, and he looked to the Holy Spirit of God to do in him what it was not at all natural for him to do. "Those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit" (Romans 8:5).
This means that setting of my mind on the hurts, failures, disappointments,
loneliness, frustrations of my life will make me wretched (Romans 7:24).
Like the world and the devil, the flesh should not be taken too seriously.
But setting my mind on the Spirit (Hebrew ruakh elohim, wind of
God) has awesome miraculous results (like raising Jesus from the dead,
8:11). Why not try this for a month and see what happens.