Am I the Problem?

There are numerous publications which tell of problems in a horse's body caused by an improper riding. Thinking that I caused Karina's problems, I stopped riding and focused on a groundwork and longeing, especially on a longeing in free canter, which is supposed to cause the release of tensions in the back muscles. So I longed and longed and waited for a miracle, which didn't happen. Karina seemed to be quite relaxed, her tail was relaxed and her breathing also, her eyes soft and friendly, but she would never lower her head, do whatever I would. Poles and cavaletti didn't help much, even longeing on hills didn't help also. Maybe she needed more time to relax, but yet I knew that sometimes she would go with her head just above the ground, in trot and canter even. It has clearly showed me that she was able to do that, but I could do nothing to maintain this. I tried different tricks, such as stopping the training immediately, hoping that she will remember what she did to make me stop the training and she'll offer me that the next time herself. I used this method on different occasions and it worked perfectly, but not this time. I know that without relaxation there is no further work and I was at a loss what to do. I tried riding again, hoping that I might help her to relax from the saddle, but it soon turned out that my weight (which is a little above 55 kilos) was causing her pain. She started running, carrying her head extremely high, bucking and kicking her belly. I could not ride. I was terrified. I had Karina examined by a specialist, who found out that nothing was wrong with her skeleton and muscles. Her back was OK and her legs and neck also. I knew exactly when did the problems begin, but that didn't help us much. I still didn't know what to do to set everything right. Fortunately, now I finally know...