I have a back brace but it does not seem to help much. More than anything it seems to be the horses way of going that determines whether my back hurts at the end of the ride. I have ridden gaited and non gaited horses and have had both types that were comfortable and both that were painful. It seems to be more about the individual horse that what breed they are.
I am getting back into riding after about ten years off and am concerned about my back. I have borderline severe scoliosis and it has always affected how I ride. There is literally no way for me to sit up straight. My right shoulder will not go back all the way unless forced which is quite uncomfortable after a few seconds and my whole core curves to the left making my right shoulder higher than the left and some of those ribs on the left side rub together. My lower back is naturally very hollowed out so that to make my lower back less curved I have to lean forward . My top half already has a natural tendency to look hunched so I make do with leaning over just a bit more so that my lower back is a bit straighter and my upper half is just a bit more hunched.
I tend to lose my balance occasionally. I think if I could sit properly it would not happen as much. When I was younger my back was not curved as much and I had less trouble. It seems the more curved my back got the more I leaned and the more I tended to lose my balance.