Buffy the Vampire Slayer (1997)

S6E17 Normal Again

Buffy is stung by a demon and suddenly experiences a jarring shift between her familiar life in Sunnydale and a mental hospital, where she's informed that the past six years have been mere fabrications of her mind. Caught in this disorienting dilemma, she grapples with the challenge of discerning what is real and ultimately faces a critical choice about which reality to accept.