A 29-year-old nurse presents to the PMHNP for evaluation after a traumatic event. Two weeks ago, she was attacked by a patient on the behavioral unit where she works. She says she is having flashbacks about the event and keeps wondering "what could I have done to prevent it?" She has had several panic attacks since the incident, and she is to the point now where she is considering quitting nursing. She says, "I guess I am too weak to be a nurse if I can't handle one attack." What is her most likely diagnosis?