So, after I wrote the post immediately below, I had dinner and did some thinking. I've come to a conclusion: All my various problems stem from a central core of doubt. This can further be broken down into two core doubts: Doubt of the world and doubt of myself.
The world doubt is probably the less dangerous of the two. It basically boils down to a doubt that the world (by which I mean both the aggregate humanity as well as the society we've built to sustain ourselves) is a healthy, stable, livable, and perpetuable place to be. Or, to simplify: We're all screwed, most of us don't know it yet, but we'll all find out very soon..
The self-doubt, however, is what's really getting me. It's pretty comprehensive; in short, I believe, in these bad moments, and probably all the time at some level, that I don't cut it. At all. Period. This manifests itself as a belief that I don't have the strength to withstand various of life's hardships, that I'm a hopeless case as a writer and just don't know it yet, or that I lack the power to effect any real change on myself and my circumstances. (Pointless digression: At fifteen, I wrote a story about a knight named Sir Cumstance. I thought this was terribly clever.) And that's the real motherfucker right there.
Hopefully, knowing this and naming it will help me move towards overcoming it in therapy. Even if it turns out that I'm totally wrong, the fact that this is what I'm thinking will at least be a signpost towards whatever the real problem is.