self-possession vs will power

self-possession and will power both are nouns.

Word	Noun	Adjective	Verb	Adverb
self-possession	Yes	No	No	No
will power	Yes	No	No	No

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

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

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

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