It would probably be a good idea to wear a helmet on your first few backflip attempts, considering that you landed straight on your head. But make sure you can backflip while standing on a trampoline without bouncing, that helped me.
Where you going for a back pretz head bonk OP? because if you were, just when you land on your head, flip the other way from the potential energy from your neck snapping. Simple stuff really, can't believe you don't have it yet...