A woman describes a recent, distressing sense of watching herself from outside her own body, as though acting on autopilot, and says this feels like a frightening change from how she used to experience herself. Before revealing the answer: is this schizoid personality disorder?

No, this is more consistent with depersonalisation than with schizoid personality disorder. Schizoid personality disorder is a lifelong, stable pattern experienced without distress; here, the recent onset, the frightening quality, and the explicit sense of change from a previous way of experiencing herself point instead toward depersonalisation, a distressing detachment from one's own self.