Ok, everything that I've ready does have some pretty accurate points. When reading the original post, my immediate thought is that the horse is trying to tell you something, there is something wrong with being ridden. When the horse feels pain, it feels vulnerable, it compromises their ability to "fight or flight", therefore their defensive behavior is going to get stronger and more sensitive. What many people see as a "behavioral problem" is that, but its not just that, that behavioral problem is stemming from defensive behavior. If the human can be an adequate leader, then the horse won't run you over, so this does need to be established. However, I personally don't want to punish my horse from expressing that I'm not doing something right. I have seen many horses behave despite their physical discomfort, and they end up just quietly going lame. I will not let any horse run me over, but I do feel that it is my responsibility to understand why the horse is being so defensive and fix that problem.
If someone sent me this horse and said "fix it", I would attack this problem from all angles. I would start by addressing the feeding program, taking ulcers into consideration. I would then get into behavioral, teaching respect, manners, and social skills putting me in the herd leader position. This is necessary to ensure your safety and the horses security and comfort. I would not get on the horses back without understanding their physical condition. I personally often start with ground work to help them use their bodies correctly, consult body workers, farriers, vets, trainers, anyone who can help me understand what is going on. Something causes defensive behavior, something causes ulcers, you have to figure out the source of the problem before you can truly fix it.
I have been put in positions where someone says "here is my pain in the butt horse, I want you to ride it". It is possible to solve a lot of problems from this angle, but it does take a lot of work to fix the horses body undersaddle. Good luck!