Could it be scoliosis?
Have you ever been checked for Scoliosis? That is a kind of twisting of the spinal column. How old are you , if I might ask? Your twist is SO obvious that I think it has to be in your spinal alignment.
Could you post some photos of how you stand and walk, when NOT on a horse?
I am going to look at the photos again, b ut this thought came to me instantley and I wanted to share this with you. Hope I did not alarm you. My son has it, friends of mine have it, it's very common.