I am getting back riding after a bit of brake when my mare got sick. While riding someone else's horse I've noticed that problems with seat sizes. I'm 5'9" and a 34" inseam but in a english saddle anything over 17" I'm swimming it in, even then I can be. Instructors and trainers look at my height and put me in a bigger saddles. Same with western really, I own a 15" saddle and it's okay but would like it .5" to 1" smaller seat so my legs are more under me.
Is it the upper part of my leg just isn't that long? The only saddle I found that I liked was a thorowgood but I have no idea what size it was, other than a 12 year old was riding in it before me (she was tall for her age). When I bought my sidesaddle the woman was surprised when I said I had a couple inches behind me in a 20" saddle, I have to skoot forward to reach the up right pommel.
Guess I should point out I am 125lbs so I am not stick thin.
Is there a type of saddle that might work better, or am I just that weirdly built

? Would like to buy another saddle but don't want to pay for something that hinders me.