Imagine you’re in charge of one of those giant computers — the kind that you see in movies. The computer contains billions of items of data that together somehow keep the Western world from falling into chaos. One day, as you watch the screen in horrified disbelief, the data begins to disappear. The world’s most virulent superbug has infected the hard drive and every fragment of information is slowly and methodically destroyed until all that is left is a blank, blinking screen.
With depression, that database is every particle of your personality. Every fragment of knowledge and experience that have provided the anchors and co-ordinates from which you are progressively able to address the world with a measure of safety and confidence, with a growing sense of who “me” is, has suddenly been completely voided. Dramatic? Yes. Terrifying? Absolutely.
This is one of the best descriptions I have ever read. It's such a hard thing to explain and I think he has hit the nail on the head.