WordNet sense 1 (destructive of morale and self-reliance):

demoralizing, disheartening, demoralising

WordNet sense 2 (depriving of confidence or hope or enthusiasm and hence often deterring action):

discouraging

WordNet sense 3 (causing dejection):

blue, dark, depressing, dingy, disconsolate, dismal, drab, drear, dreary, gloomy, grim, sorry, dingy

WordNet sense 4 (lacking cheer; depressing):

uncheerful