I have diagnosed GAD. What i usually do in during a panic attack is try to rationalize. What i do is try to think of the odds. Like for instance i'll go through things in my heads. For example i'm driving down the highway, how high are the odds that i'll die in a car crash? Slim to none right? So think of it like that maybe. Think to yourself what are the actual odds of you dying in a dark room? Highly slim to none. Just take a deep breathe. It'll pass.