A 17-year-old girl is brought to the office by her parents due to recurrent episodes where "she seems to be lost in her own world." The parents say that the episodes began about 2 years ago, but seem to be occurring more frequently since a difficult breakup with her boyfriend several months ago. The patient acknowledges that she seems to "space out" easily. She says, "I often feel numb, like I'm in a fog or a dream. Sometimes, I feel as if I'm not myself, as if things are happening to someone else." The patient uses nonsteroidal anti-inflammatory drugs for menstrual cramps. She has no other medical conditions. She smokes cigarettes but does not use alcohol or illicit drugs. Physical and neurologic examinations are normal. On mental status examination, the patient is cooperative with a mildly anxious mood and flat affect. Which of the following is the most likely diagnosis?