Depression isn't feeling like nothing, it IS the nothing. It's the nothing in the air, the nothing in "good mornings" and "good nights". It's the nothing in your life, and it's the nothing that will be your death.
And you know there used to be something, because you used to feel that something, but now it's suddenly the nothing. So was the something ever even there or is the nothing waiting to be something?
And you panic, because all you know now is the nothing. And as you panic, you fall further into this nothing abyss.
And you don't feel dead, but you certainly don't feel alive. You're floating in the nothingness, screaming for someone to somehow pull you back into something.
But they can't, because all they see are the somethings, and all you have are the nothings.