WordNet (r) 3.0 (2006):

self-doubt
    n 1: lack of self-confidence [syn: diffidence, self-doubt,
         self-distrust] [ant: confidence]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "self-doubt":
   Pyrrhonism, apprehension, diffidence, distrust, distrustfulness,
   doubt, doubtfulness, dubiety, dubiousness, half-belief,
   lack of self-confidence, lack of self-reliance, leeriness,
   misdoubt, misgiving, mistrust, mistrustfulness, question, scruple,
   scrupulousness, self-depreciation, self-detraction, self-distrust,
   self-effacement, shadow of doubt, skepticalness, skepticism,
   suspicion, suspiciousness, total skepticism, uncertainty, wariness,
   weak ego