"Difficult" or not isn't really the question, Some people have it more than others. I have been training horses, ponies and minis, working with rescue horses, off track horses, wild mustangs, starting young horses, riding, driving etc.. for over 12 years and find it comes naturally to me. What I cannot do is give a riding lesson to save my hide! I do not have the patience for people but I can work with any horse.
I did work under several riding instructors and just observed every little thing they did, every one does something different. I watched and decided what I liked, what I did not like, what would work for a quiet QH may not be the way to handle of OTTB. I read everything, I've watched different famous horse trainers and there different ways, Just soak up everything you can and then work with a horse. Don't be afraid to ask questions, always work from the ground up and remember every time you are with your horse, labeled "horse trainer" or not, you are either training or un-training your horse!