willpower vs self-possession

willpower and self-possession both are nouns.

Word	Noun	Adjective	Verb	Adverb
willpower	Yes	No	No	No
self-possession	Yes	No	No	No

As nouns, willpower and self-possession are synonyms defined as:

willpower and self-possession: the trait of resolutely controlling your own behavior

Other synonyms of willpower include possession, self-command, self-control, self-will, will power.

willpower (noun)	self-possession (noun)
the trait of resolutely controlling your own behavior	the trait of resolutely controlling your own behavior