I thought I'd share a bit of my experience.

It's hard to adequately explain to someone who doesn't have this kind of depression just how it feels. I can be going along in my own little world, feeling perfectly fine - I don't like using words like "happy" or "content" because I'm not sure what they mean - and then a dark cloud appears on the horizon of consciousness. I gradually become aware of the black fringe on this margin and try to deny its existence but all the denial does is allow the depression to assert itself and move further in. It is at this point that I would, if I could, stop it in its tracks and send it back to whatever hell it came from. But I just can't and the sense of the black cloud slowly overcomes everything else, a darkness overlaying the light. Once it's there, it only shifts when it wants to shift and no attempts on my part have any effect.

The impact this has on me is twofold. Firstly, it makes it impossible for me to think of anything except the black cloud which has appeared and won't disappear. Every other activity is impacted and every single potential for a moment of pleasure or joy is spoiled because it is filtered through the overwhelming feeling. Secondly, it makes me excessively inward looking and suspicious, feeling as if no-one has ever felt as bad as I do and thinking that everyone who knows me must find me just as tedious company as I find myself. Inevitably, this is a self-fulfilling prophecy and the more I become convinced that people dislike me, the harder to like I become. I can sometimes see it in peoples eyes that "he's off again" and I try desperately not to communicate my feelings but they seem to have a habit of somehow slipping out. This becomes another thing to add to the black cloud and confirms to me that I am, fundamentally, not worth a great deal.