I'll give it a shot.

The really painful part comes from wanting to do so much while being paralyzed. People have told me to get myself checked out, but I was in therapy for a while and it turns out I'm perfectly healthy - just anxious, and mildly neurotic.

Having had those periods, thankfully brief, it's all too easy to fall into the spiral of "Another hour/day/week wasted, why a sorry sack I am", while not yet feeling, "I need an intervention". Maybe you (or I, or anyone in a down mood) really do need outside help. But while you debate that, exercise is, for the most part, something you can control. And if you haven't exercised in awhile (which is for me, basically any given day)...you can also benefit from the phenomenon of "well, something is being done, so things must be changing for the better"* (even though you're the initiator of the change). Really, the most important thing is that it's something within your control.

* there's a name for this...it's related to some study where researchers kept repainting the walls of a factory and observed workers becoming happier, regardless of what the color was...

The name is "Hawthorne effect".

This is great advice. I know it seems weirdly unrelated to your mood but it has helped me from the same sort of mood issues. Let us know if it helps=).