mental strain vs nervousness

mental strain and nervousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental strain	Yes	No	No	No
nervousness	Yes	No	No	No

As nouns, nervousness is a hypernym of mental strain; that is, nervousness is a word with a broader meaning than mental strain:

Other hypernyms of mental strain include nerves.

mental strain (noun)	nervousness (noun)
(psychology) nervousness resulting from mental stress	a sensitive or highly strung temperament
	the anxious feeling you have when you have the jitters
	an uneasy psychological state