I Jam on my Back handsprings?):?

Hey. Im 12, & I have a tumbling cl today, my second one at American Pride. I haven't been to a tumbling cl in weeks, so my tumbling is going to be a little rough): But for some reason, I have started jamming on my roundoffbhs. It hurts, & I'm scared to do it): Any advice on how to stop jamming, & how to get ovr my fear? also, on my standing, I always undercut soooooooo bad. How do I lengthen my standing bhs? And, I havent done any tucks or roundoff bhs tucks in a loooong time, & I rly suck at them cuz I CANNOT set!! i try, but I just bend my arms and cant raise them above my head, touching my ears. I just go straight back. help?! Also, how do i get more power on my tumbling?? my friend has sooooooo much power, but I don't): Thanks!!!(: