downhearted vs down in the mouth

downhearted and down in the mouth both are adjectives.

Word	Noun	Adjective	Verb	Adverb
downhearted	No	Yes	No	No
down in the mouth	No	Yes	No	No

As adjectives, downhearted and down in the mouth are synonyms defined as:

downhearted and down in the mouth: filled with melancholy and despondency

Other synonyms of downhearted include blue, depressed, dispirited, down, downcast, gloomy, grim, low-spirited, low.

downhearted (adjective)	down in the mouth (adjective)
filled with melancholy and despondency	filled with melancholy and despondency