First off, thank you all so much for the advice! I'm female and probably do feel some undue gender-related pressure in addition to the impostor syndrome. I am glad to know that I'm not alone in the feelings.
I actually spent two days trying to make sure I didn't accidentally mark the wrong ethnicity on my form because I got an email with "Presidential or Diversity Fellowship" in the title. (I would HATE to show up on admit day only to reveal a glaring error I had made in checking selection boxes.) I have since concluded that these people know what they're doing. In fact, I'm they have a reputation for being highly rational, if nothing else. (Like robots.) I'm sure they checked up on me. (Especially considering one LOR writer tracked me down and told me off for being so lazy as an undergrad/made me swear I'd be better next year.)
Second, I guess I should be grateful that, last year during my MA, classmates and professors thought I didn't have it in me to succeed. I had to get scrappy. I even learned how to slowly build a study group, by tutoring the weakest person in the class, who then invited a friend, etc. until all but 3 of the whole cohort were studying together. (I drew a circle that drew them in.) I'm building my artillery now to fight off all the internal and external doubts that are inevitably on their way. First step: learn computer programming.
qbtacoma, I also withdrew during adolescence (6th grade). I feel like American youth culture is increasingly philistine. (Don't know if you're from the US so maybe it's more than a domestic phenomenon?) I must have been in my own little bubble by the time everyone started caring about grades because I never developed that opinion that "knowledge" is a zero sum game.* While I was going through the "all rejection" phase of this application period, it finally occurred to me that what I truly crave is intellectual stimulation and camaraderie. I hope that I find it, either in grad school or because of grad school.
*And law/business schools who still function that way can suck it. Intellectual mercantilism is an obsolete idea.