I agree with all of the above, but I'd like to throw something else out there.
In some cases, it is mind over matter.
I had a tendency to just fall off, even when the situation wasn't that bad. Balance was part of it, but a big thing was confidence.
Then, one day, I took a horse out for a ride in a pasture. He decided he didn't really want to play anymore, so he whipped around and took me for the ride of my life. At that point, he was going so fast that I figured I'd probably die or be seriously maimed if I let go, so it was enough to keep me on. Now I'm at the point where it's pretty hard to get me off.
So, not too much help, but consider confidence!!
Riders aren't 16 and pregnant. Riders are 16 and arthritic.