The Collaborative International Dictionary of English v.0.48:

Dishearten \Dis*heart"en\ (d[i^]s*h[aum]rt"'n), v. t. [imp. & p.
   p. Disheartened (d[i^]s*h[aum]rt"'nd); p. pr. & vb. n.
   Disheartening.] [Pref. dis- + hearten.]
   To discourage; to deprive of courage and hope; to depress the
   spirits of; to deject.
   [1913 Webster]

         Regiments . . . utterly disorganized and disheartened.
                                                  --Macaulay.

   Syn: To dispirit; discourage; depress; deject; deter;
        terrify.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

disheartening \disheartening\ adj.
   Causing loss of hope or enthusiasm.

   Syn: demoralizing, demoralising, dispiriting.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

disheartening
    adj 1: destructive of morale and self-reliance [syn:
           demoralizing, demoralising, disheartening,
           dispiriting]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "disheartening":
   alarming, awing, black, bleak, chilling, daunting, depressant,
   depressing, depressive, despondent, deterrent, deterring,
   disappointing, disconcerting, discouraging, dismal, dismaying,
   dispiriting, displeasing, disquieting, dissatisfactory, down,
   dreary, fear-inspiring, fearful, fearsome, frightening, frightful,
   funereal, inadequate, incommensurate, insufficient,
   not good enough, oppressive, overawing, pessimistic, scaring,
   scary, somber, startling, unfulfilling, ungratifying,
   unsatisfactory, unsatisfying