I didn't read everything everyone posted on the topic of trying to stop your horse, but I will offer my experience with a extremely fast and dangerous barrel horse.
She had been previously run on barrels by owners, and I did not know those owners so couldn't tell you more about her background. The only thing I knew for sure is that she was extremely smart, and when you climbed on you had better be ready to run. If not, she would crow hop, buck, rear, you name it.
We tried every training metod we could think of to get her to slow down. We even brought in a trainer to work with her. We tried starting at the walk (she did this very well....and whoa'd well from here)..ground manners were excellent....lunging came along wonderfully. BUT...the riding never changed.
The trainer recommended that I either take her to the barrel race and race her (and she would win!) or to sell her to an experienced barrel racer who could put her to good use.
I wanted to race her myself, but didn't have the means to travel to the rodeo so I decided that it would be better to sell her to someone who could enjoy her and vice versa.
It was an act of God that the first barrel racer who came to see her was a PERFECT match! She is at the state level for racing - and needed another horse. Even she agreed on the first day of riding her that this would never be a horse for anything besides racing once per weekend.
Turns out that the horse absolutely loved racing. She is racing and doing great now....we are all happy :).
My point is, pay attention to what your horse may be and where she came from....maybe she only has the heart to do one thing.