down in the mouth vs downhearted

down in the mouth and downhearted both are adjectives.

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

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

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

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

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