rigorousness vs difficultness

rigorousness and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
rigorousness	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficultness is a hypernym of rigorousness; that is, difficultness is a word with a broader meaning than rigorousness:

rigorousness: something hard to endure
difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of rigorousness include difficulty.

rigorousness (noun)	difficultness (noun)
something hard to endure	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
excessive sternness