The Queen called 1992 the worst year of her reign because everything went wrong. Princess Diana died in a tragic car crash in Paris with her lover Dodi Al Fayad. Queen Elizabeth received a lot of vitriol and criticism for her reaction to this event (via Town And Country Mag).
She made an infamous speech that came across very badly in the U.K. This was the only time of her reign where people seriously criticized her on a personal level. However, she rebounded and rehabilitated her image for a new generation. But this was a crucial lesson for the veteran monarch.