As tragedies go, not getting what you want is the straightforward kind, and getting it can be the ironic variety. But there is also the existential tragedy of not knowing what you want to begin with. That’s the species of catastrophe recounted in Walter Kirn’s memoir, “Lost in the Meritocracy: The Undereducation of an Overachiever,” the witty, self- castigating story of the author’s single-minded quest to succeed at a series of tests and competitions that took him from one of the lowest-ranked high schools in Minnesota to Princeton. As Kirn, a noted critic and novelist, tells it, in childhood he leapt onto a hamster wheel baited with “prizes, plaques, citations, stars,” and kept rattling away at it until his junior year in the Ivy League, when he suffered a breakdown that left him nearly speechless.