I'm seriously depressed and only just realized it.?

I've been feeling this way gradually for a while now, but I've been denying it to myself. I think there's a part of me that thinks that it isn't okay to not be okay, you know? I'm nearly always cheerful by default, but lately it's been getting almost hysterical or maniacal, and I'm starting to realize that this may be covering up a deep depression.

I was really depressed two summers ago (for several really good reasons) and I thought I was over all that, but maybe I'm not. I feel like life is a merry-go-round and I'm in the center of it, and I'm trying to jump in, but it's whirling by so fast. All I want is a stupid ceramic horse to sit on, but I can't find a moment to leap into the rhythm of the whole thing, and it's just going on without me, and I'm just sitting in the center, watching the world go by while I passively miss people that I used to know.

I think I've been in denial, but now I'm realizing that I'm actually quite miserable. How do I get out of this funk? I can't spend a lot of time with friends - part of the problem is that my social and family lives are sort of in shambles, and I only have one friend who lives nearby, and he can't babysit me 24/7. What do I do? :(