Well... just self-reflection. :D Thank you for the comment :) Yes, it is hard the academic definition to catch the feeling. In this connection I want to share with you one poem by Edgar Allan Poe about depression:

“Alone”

From childhood’s hour I have not been
As others were—I have not seen
As others saw—I could not bring
My passions from a common spring—
From the same source I have not taken
My sorrow—I could not awaken
My heart to joy at the same tone—
And all I lov’d—I lov’d alone—
Then—in my childhood—in the dawn
Of a most stormy life—was drawn
From ev’ry depth of good and ill
The mystery which binds me still—
From the torrent, or the fountain—
From the red cliff of the mountain—
From the sun that ’round me roll’d
In its autumn tint of gold—
From the lightning in the sky
As it pass’d me flying by—
From the thunder, and the storm—
And the cloud that took the form
(When the rest of Heaven was blue)
Of a demon in my view—