The Collaborative International Dictionary of English v.0.48:

Diffidence \Dif"fi*dence\, n. [L. diffidentia.]
   1. The state of being diffident; distrust; want of
      confidence; doubt of the power, ability, or disposition of
      others. [Archaic]
      [1913 Webster]

            That affliction grew heavy upon me, and weighed me
            down even to a diffidence of God's mercy. --Donne.
      [1913 Webster]

   2. Distrust of one's self or one's own powers; lack of
      self-reliance; modesty; modest reserve; bashfulness.
      [1913 Webster]

            It is good to speak on such questions with
            diffidence.                           --Macaulay.
      [1913 Webster]

            An Englishman's habitual diffidence and awkwardness
            of address.                           --W. Irving.

   Syn: Humility; bashfulness; distrust; suspicion; doubt; fear;
        timidity; apprehension; hesitation. See Humility, and
        Bashfulness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "diffidence":
   Pyrrhonism, afraidness, apprehension, bashfulness, boggle,
   boggling, caution, cautiousness, compunction, demur, demurral,
   distrust, distrustfulness, doubt, doubtfulness, dubiety,
   dubiousness, falter, faltering, fearfulness, half-belief,
   hesitance, hesitancy, hesitating, hesitation, jumpiness,
   lack of self-confidence, lack of self-reliance, leeriness,
   misdoubt, misgiving, mistrust, mistrustfulness, modesty, objection,
   pause, protest, qualm, qualm of conscience, qualmishness, question,
   recoil, scruple, scrupulosity, scrupulousness, self-depreciation,
   self-detraction, self-distrust, self-doubt, self-effacement,
   shadow of doubt, shilly-shally, shilly-shallying, shrinking,
   shrinkingness, shyness, skepticalness, skepticism, skittishness,
   stage fright, startlishness, stickling, suspicion, suspiciousness,
   tentativeness, timidity, timorousness, total skepticism,
   uncertainty, wariness, weak ego