I was going to suggest "drink heavily" until I saw her age!
I flew the first time when I was 11 and fell in love with it, until........Turkish Airlines domestic in the 1980's. The pilots still thought they were in their F5's or something as they would ascend/descend at max velocity and put the thing on their wing tips every now and then, 1 out of 3 flights I seriously thought we were in trouble.
It was years before I wasn't very anxious during flying, especially during turbulence and would get well lubricated while flying in order to be relaxed. I know it's not much help to a 16 yo girl but sometimes you just have to buck up and do what you need to do.