Help with Back Handsprings?!?

Well, i'm almost 13 and I want to know how to do a back handspring. I've been looking up videos of them for the past 3 days, but every time I try my handspring I do the thing where your sitting in a chair position, then when I'm about to fall I jump, but I just fall. I keep my arms straight, but maybe it's because i'm not jumping high enough or because my back isn't arched enough? I've been trying this on my trampoline for about 2 days and I can do a back flip on my trampoline, a handstand, and a bridge standing up. I am also very flexible. Please help mee!