Participating in horse races in The Witcher 3 is a great way to earn coin and get upgrades for your horse. Most of the races are easily won. But the Palio is set up to be incredibly difficult. It seems like it's outright rigged from the start. The AI cheats by seemingly having unlimited stamina and by actually trying to force you off the track, resulting in an instant loss. I endured several losses before settling on a strategy to sprint in the open and cantor across the bridges (thus replenishing Roche's stamina). Even then, I'd get waylaid by a wraith or a cockatrice. Shoving off for Skellige before running the race so that I could buy the zerrikanian saddle made the difference in the end, but I still had to make sure to stick to the track or end up getting instantly disqualified.
Of course, if you're lucky, you might just be able to take advantage of a glitch where you'll have no competition at all in the Palio race. I don't know if it can be replicated on Xbox One or PC, but on PS4 make sure you save your game right before you talk to Cleaver about the letter he sent you. Every so often, the Palio will load and one of the competitor horses won't have a rider. If you keep loading Witcher 3 back to where you ask Cleaver about the letter, you'll eventually get a version where you start the Palio with both competitor horses not having riders. Once you have that, you can just cantor the whole race and finish it at your own pace. Here's a video of me winning the Palio race in The Witcher 3 because of the glitch: