How can i get my horse to stop bucking when i try to make her go faster?

I have a 7 year old quarter horse mare who is bred for penning, and roping. She is trained for barrels, poles, etc. and when i try to get her to go faster than a gallop she bucks. One time i rode her out of the arena(in our pasture) and tried barrels at a canter and i tried to get her into a gallop and she started to bolt and buck, i ended up getting a concussion. So PLEASE help me get her to stop because i know that she can do it!