If you ride horses please help me!!!?

Okay, i am 14 years old. I have had a horse since halloween ( for about 4 or 5 months), I would say im a pretty good rider, but i never can not hang on. Ive tryed relaxing and everybody sayes Buck (my horse) has a really smooth canter, but i never can seem to not hold on to the saddle horn or something. I played sports and everything im physically fit, and i relax, but why can i stay on?? Oh and i want to start barrel racing soon, any tips?? And any tips on how to Gallop, i never have galloped either... Oh and buck was a roping horse, so hes a BEAST!!! please help me!!!! :) thanks, no rude comments please.