do you have a photo of yourself in this "chair seat"? Many times a chair seat is a result of a saddle that doesn't fit the horse very well or is placed too far forward. If the saddle is angled too much upward in the pommel, then you are forced to lean forward, I.e. Lean up into the hill . If you have your upper body forward but your butt is falling back into the cantle of the saddle, your natural tendency will be to put your lower legs forward to counterbalance the upperbody. One would think that you would put them BACK but it is not so. You will have to struggle to bring them back under you and as soon as you start trotting, you will naturally find them creeping forward, until they are under your upper body. So, look at your saddle. Western? English?
You may need to bring it a bit further back (many dressage riders put theirs too far forward) or, if the horse has a back that causes this angle, then add a small lift under the back of the saddle , like a Cashel wedge. See if that doesn't feel different. If you want a temporary trial, you can arrange some folded towels in layers that gradually taper in thickeness to provide a lift to the back of the saddle without an abrupt edge thatq the horse will feel under the saddle. If that feels better you will know that a wedge is your solution.
Let me know how that works. I am curious. And can you post a photo?