Terrified is fear at its highest setting — the kind that empties your head and locks your legs. A child terrified of the dark, a passenger terrified in turbulence, the face of someone who has just heard a noise downstairs.
It takes OF for the thing you fear (terrified of spiders, terrified of flying), THAT for the thought (terrified that she would find out), and TO for the thing you cannot make yourself do (terrified to look down). Keep it apart from terrifying: the film is terrifying, the audience is terrified — the film does it, the people feel it. Say I was terrified, not I was terrifying, unless you really were the frightening one. Its family is terrify, terror and terrific, and terrific walked off in the opposite direction to mean excellent.