Stress
The stress of winning is huge, maybe even more so than the stress of falling off.
We're just kidding. That "falling" stress is always serious and always present.
The stress is definitely real, as your career depends on you winning, and winning consistently. If you lose you'll make no money and will be branded a loser (and who wants that?). After all, many people—including the owner of the horse and the people betting on the race—will have sunk big money into the horse you're riding.
Of course, if you push it too hard or some kind of accident happens, you could fall off and maybe even break your neck. So, basically, there's pressure to not do too much and also to not do too little. Perhaps some yoga or breathing exercises will help calm you down the night before the big race.