A 35-year-old female with a history of severe childhood sexual abuse presents to a psychiatric clinic after referral from her therapist. The therapist reports observing distinct shifts in the patient's demeanor, voice, and behavior during sessions — at times she presents as a confident, assertive adult, and at other times she regresses to a childlike state with a different name, different handwriting, and no recollection of what occurred minutes prior. The patient reports frequent gaps in memory for everyday events, finding items she does not recall purchasing, and being told by others about conversations and behaviors she cannot remember. She denies substance use. Neurological workup including EEG and brain MRI is unremarkable. The most likely diagnosis is: