Which word describes someone who has lost their self-possession?

The term that describes someone who has lost their self-possession is "disconcerted." When a person is disconcerted, they experience a sense of confusion or distress which can lead to a loss of composure and self-control. This emotional state can result from unexpected incidents or information that trouble the individual, effectively disrupting their calm demeanor and steadiness.

In contrast, the other words provided convey different meanings. "Confident" refers to a sense of self-assurance and belief in one's abilities, which is the opposite of losing self-possession. "Assertive" describes someone who expresses themselves effectively and stands up for their point of view, also indicating strong self-possession. Lastly, "independent" signifies a person’s ability to think and act on their own, which entails a degree of self-control and confidence. Thus, "disconcerted" is the only choice that directly aligns with someone who has lost their self-possession.