Certainly examine and try the things that have been suggested. But I'll bet money you sit the horse crooked. I'll also bet that you walk crooked. I'll also bet you have one leg that is shorter than the other. And finally, I'll bet the horse is crooked in the same way you're crooked.
Have yourself examined by a healthcare professional to evaluate your own crookedness and come up with a plan to even yourself out. Once you're straight, you can then work on straightening the horse.