For me, when it's really, really bad:

It is like I am trying to live in another world, an evil, black, cold, hellish place devoid of peace or love or caring or life. Loss of hope, anguish, shattering physical exhaustion, no interest in or motivation to do anything but sleep, no appetite, cannot do any activity of daily living, won't answer phone or door or text, care about absolutely nothing, excruciating abdominal and chest pain that never relents, confusion, and a strong desire to be dead immediately. All of this with the firm and deep belief that it will never, ever, ever stop.