Been there for quite a while myself, friend. It's basically exactly as you described it, although at times you get a bit of shame and unrelenting guilt or grief that seeps its way in and stirs up the unending bitter melancholy.

The pointlessness--that's what gets to you after a certain point, when your brain is chemically as unbalanced as it can get before breaking. You've gone through all the "problems" that have brought you to depression, and which continue to keep you here, and then you start to rediscover very simple, basic things, things like eyes, and thoughts, and concepts, and fuck, existence... And you ask yourself, why? And after that, why this? I'm thinking. I'm conscious matter, Goddamnit! I am experiencing and apparently alive, a living entity, and this is the world? This asinine, pointless shit, with all its problems and assholes and beauty and wonder which we so eagerly want to just fuck up? But you're too buried under miles upon countless miles of grey muck to really do much else but look at it.

You become frozen. Happiness comes in small delights, if at all. Here and there, little spurts of something not entirely wretched, a foolish smile, a stupid joke, but even then, that doesn't counterbalance what has, at this point, quite literally become you. You wonder what happiness means. You wonder how any of us is or can ever really be happy. All the while these thoughts are just mulling about on the surface in an incoherent cloud--they're there, certainly, but they've become more than that, they get into your gears, they clog you up. You might remember what first got you here, to "depression"--but now all that seems so far away, possibly silly, even, because you've grown so distant from it, you've found new holes to burrow into and dark little crannies into which you can sink your fibrous roots, and now you're trapped in this place. Everything outside loses its light, eventually. Maybe you can still see some of it, in the glow of people's faces or a good movie or the stars at night when you're not near a bright city, but it's out of your reach.

It should go without saying that all this time you've been fantasizing about your death; there are other things, normal fantasies, and some more specific to you, but it all comes back to that, to death. At first it's almost exciting, guilty though it makes you feel. It starts out as a story, an I would never really go that far, ha ha and suddenly it becomes the only remaining option, now it's just a matter of when, and how. Then the theatrics stop. The self-pity and loathing stop. The worries vanish. The dream ends and you wake up, only it's not what you think. Now it's real, and here you are, and God how could I have fallen so fucking far...? Only there is no God, of course, how could there be? And you come right back to your initial rediscovery of the basics, to your first, deep why. But this time, it's not why this society or why this set of physical laws, or even why me. This time it's why not. Why Not nothing? What matters? And if nothing, why not become that? What began as symptoms of something swallow you up, like gravity, and you've burned through all the heavier elements right up to and surpassing iron. You strip away everything that gave the world order and meaning, but even then, the pain never goes away.

I guess it's a little like that.

Edit: some grammar and I'm sorry for such a long post