Quote Jung on depression: at mid-life,

 when a metanoia, a basic reorientation of one’s attitudes and goals is called for. [the opposite of adolescent patterns is called for]...

it becomes necessary to follow the depressive tendency

 into the unconscious..

.and discover where and how the libido[vital life force] wants

to flow again into the world. 

This requires two things:

(1)   the willingness to let go

of one's habitual conscious attitudes and assumptions,

particularly one's so-called "reasonable" convictions about the world;

and (2) the overcoming of the fear of incest,

 i.e. the repulsion and anxiety

 that contact with the unconscious calls forth.

 …every significant change in age, status, role, attitude, or personality

 is accompanied by the demise and mourning of a former condition.

  Only through such "dark nights of the soul"

 is the person reborn into a new way of being and living."

"in its natural condition

the unconscious is in a depressed state. 

The usual symptoms associated with depression-

-the feelings of inadequacy, inertia, heaviness, sadness, blackness,

 lack of interest in life and the pull towards death-

-are apt descriptions of the lower depths of the psyche.

 It is no wonder that consciousness,

 normally activated by the opposite principles-

-spirit, light, energy, joy, curiosity, life—

fights vigorously not to fall into the hands of the unconscious.

 ...(P.339)the unconscious deplores its depressed condition

and longs to be made free of it,

and that within its blackness

contains a germ of consciousness

capable of unifying the conscious and unconscious parts of the psyche

thereby healing the split soul of man.

 ...Jung's ideas about the nature of depression...

 he thought it was a compensatory response by the unconscious

 to one-sided or erroneous conscious attitudes and behavior.

  Next, he saw depression and mania

as two possible reactions

 on the part of an ego-consciousness

 that has become aware of the powers of the unconscious

 and feels its stability and formerly assumed superiority threatened....

  He described depression as

 an aspect of the regressive tendency

 associated with the incest longing [the desire to return to the womb p. 344]

 regression occurs because

 an obstacle is encountered

 in conscious development or social adaptation

 and there is a need for a renewal and a rechanneling

of the flow of psychic energy."