Explain why derealisation does not mean a person has stopped believing in objective reality.

Insight is retained throughout: the person knows, at a factual level, that the world has not actually changed — what has changed is only how it feels to them subjectively. They are describing a disturbance in their own experience, not asserting a belief that the world itself is different. That retained insight is exactly what keeps derealisation distinct from a delusional belief about the world, where the person would hold, with genuine conviction, that reality itself had changed.