It is really hard to describe to people what this feels like. I have heard such ignorant comments as "You laugh all the time, you can't be depressed", or "Ha! I made you smile, you're not depressed".
Probably the easiest description is to picture waking up just before sunrise on a really cloudy and overcast day. There is light in the sky, enough that you can walk around without a need for a light, but everything is darker than it should be. You can't wake all the way up, even if you aren't tired. If you are having a really good day, the sun comes over the horizon, but never clears the thick clouds, and you can't shake that sleepiness. On the bad days, you hope that the rain comes, and takes you away from everything, but it never does. The world has no joy, not for you. Colors are duller, the things you love are boring, food all tastes bland, you want to do nothing but sleep, you aren't tired, and nothing will ever get better.
Before I got really bad in 2009, Allison pointed out to me that I was acting like I was depressed, so I got some Celexa from my Primary Care Physician. It was enough of a dose to clear the clouds, I could function as a human being. I didn't notice until far too late that I was still suffering from symptoms: mostly attention span. This let my work suffer, which I have summarized elsewhere. When everything finally hit me at once, I started being more proactive about what I need to do to take care of myself: get to a psychiatrist and a therapist. In the first week of my LOA, I got into see both of these professionals, the Psychiatrist upping my mediation by 50%, and the therapist giving me a good outside perspective on myself, one that I really needed.
I don't have too much more to say on this, but if you ever find yourself feeling like you can't go on, please get help. I know it is hard, but you need to take care of yourself, and you can't always do it alone. Trust me.