severeness vs difficultness

severeness and difficultness both are nouns.

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

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

severeness: 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 severeness include difficulty.

severeness (noun)	difficultness (noun)
extreme plainness	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
excessive sternness
something hard to endure
used of the degree of something undesirable e.g. pain or weather