In full control of one's faculties, and having a firm belief in one's abilities; confident, assured and poised(adjective)
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Use "self-possessed" in a sentence
"The very young, self-possessed, blond girl in the bright red sleeveless dress in "Glass Door" 2006 sits on the floor with her back to a window."
"So throughout my journey, I definitely identified much more closely with the struggling hero archetype than with the self-possessed goddess archetype."
"After grabbing glasses of water, I began asking her questions ranging from how she balances work and family to how she became such a self-possessed businesswoman."