I received the nicest horse I ever owned when a neighbor decided she "hated him". He was a pill on the ground, pinned ears about anything touching him, but was built like an Advanced Eventer & rode like a dream!!! She eventually regretted the decision when she realized how far I was getting riding him. Her problem was she took it personal - all his attitude & ear pinning. To me, horses just don't think that way, they aren't out to "get you".
So anyway, make sure you aren't making the horse worse by letting your own attitude affect how you handle & ride him.
That said I had a horse that didn't like the way I rode or handled him. It was a classic personality clash. I struggled with him for 3 years, and when I finally gave the reins to someone else, wish I'd done it 2.5 years sooner.
But I was the owner of that horse, and supposedly riding for fun.
As others have said, if you are an employee, and as long as you aren't scared or feel unsafe on this horse, get your legs around it and get the job done. You'll be rewarded later for being the employee willing to do whatever needs to be done, and it's a good exercise in self discipline!