A 24-year-old female graduate student presents with persistent distressing experiences that have been occurring daily for the past 8 months. She describes feeling as though she is watching herself from outside her body, as if she is in a movie or a dream. She states that her surroundings frequently feel unreal, foggy, or two-dimensional, and that familiar places sometimes appear strange or artificial. She reports that she knows these experiences are not actually real and that she is not truly detached from her body, but they cause her significant distress and difficulty concentrating on her academic work. She denies any history of trauma, substance use, seizure disorder, or migraine headaches. Her mental status examination reveals an anxious but fully oriented individual with intact reality testing, logical thought processes, no hallucinations, no delusions, and no suicidal ideation. MRI and EEG obtained prior to referral were unremarkable. What is the MOST likely diagnosis?