letdown vs frustration

letdown and frustration both are nouns.

Word	Noun	Adjective	Verb	Adverb
letdown	Yes	No	No	No
frustration	Yes	No	No	No

As nouns, frustration is a hyponym of letdown; that is, frustration is a word with a more specific, narrower meaning than letdown:

letdown: a feeling of dissatisfaction that results when your expectations are not realized
frustration: the feeling that accompanies an experience of being thwarted in attaining your goals

Other hyponyms of letdown include defeat.

letdown (noun)	frustration (noun)
a feeling of dissatisfaction that results when your expectations are not realized	an act of hindering someone's plans or efforts
	the feeling that accompanies an experience of being thwarted in attaining your goals
	a feeling of annoyance at being hindered or criticized