Both Don't be discouraged and Don't be disappointed are perfectly natural things to say, and in many contexts they'll effectively mean the same thing - speaker is advising someone to look on the bright side (to find good things in a bad situation). As OP has discovered, the dictionary definitions are somewhat different, but they're obviously closely related. A "defeatist" reaction to finding ...
Your question falsely assumes it's one or the other. A describes the mood or state of the people when they went away because "disappointed" is an adjective, which modifies the noun "people," like one pictures them crestfallen. B describes the manner in which they went away because "disappointedly" is an adverb, which modifies the phrasal verb "went away," like one pictures them leaving looking ...
22 To say the actors were disappointed in their director is an understatement: a director who is visibly bored by his cast and their performances is hard to suffer. Disappointed in is the only possibility for the first due to the use of understatement. Suffer is used less often than its synonym tolerate. The other choices don't really make sense.
The Business Journals: Journal Profile: Chef Amanda Turner forged her own path to become a culinary star
Journal Profile: Chef Amanda Turner forged her own path to become a culinary star
May Sarton was a novelist and an avid keeper of journals, but she considered herself a poet above all else. Novels and journals, she said in 1983, are concerned with growth over time, but “the poem is ...