Imposter Syndrome

I don't know how many of you understand high-pressure situations like a PhD program, but "Imposter Syndrome" is actually pretty common. It's the belief that you shouldn't be where you are, that you can't compete with your colleagues, and that your place among them is some kind of clerical error (in the words of one professor, who was actually describing a hypothetical B+, "they mailed your acceptance letter by accident").
Most graduate students feel IS at some time, (unless you're either Becky or Ryan, the two most suitable, confident scholars I know) and mostly, it goes away when the anxiety wears off. For the most part, we're all where we fit, and if we don't fit, the rejection will drive us away fairly quickly. Sometimes, the only thing we need to fit in is the same level of determination.

Two horrific things complicate my case of IS.
One of my good friends and fellow scholars did so badly on a paper once that his professor LITERALLY told him he wasn't sure how he'd even gotten into the Masters program. The horror of the situation is not his rejection, but that secretly, I believed that professor. I do not think that person, as much as I love him, is suited to academic life or thought. And that mental betrayal is one great key to my own insecurities.

The second key to my insecurity is a deep familiarity with my weaknesses. I'm not lazy, but I do not estimate well time required for a project, and I cannot figure out how to give myself time to draft, or hold myself to an arbitrarily early writing schedule. I'd love to have all of my seminar papers written a month early, and I could take that month and hone them down until they were something I could be proud of, no matter what the grade.
But I can't, so my papers are inevitably finished mere minutes before they're due, about two drafts short of my own expectations, and so when they receive a grade I always feel that my professors have been tolerant and generous - I never get to feel self-righteously indignant that my efforts did not return well-enough.

I forgot to mention a third key to IS insecurity: I suffer from crippling anxiety. But that's neither here nor there, and I can't afford the medication.

My process is broken, and I am incapable of fixing it. My own determination is not enough. I only have one semester of coursework left, but what about my reading? My dissertation? My career? Will I look back at my inadequate study habits and blame them for having to teach introductory-level rhetoric courses for the rest of my life? Will I be able to adjust to an almost completely internally-regulated system?