downheartedness vs unhappiness

downheartedness and unhappiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
downheartedness	Yes	No	No	No
unhappiness	Yes	No	No	No

As nouns, unhappiness is a hypernym of downheartedness; that is, unhappiness is a word with a broader meaning than downheartedness:

Other hypernyms of downheartedness include sadness.

downheartedness (noun)	unhappiness (noun)
a feeling of low spirits	emotions experienced when not in a state of well-being
	state characterized by emotions ranging from mild discontentment to deep grief