Get an independent saddle fitter out, watch, ask questions, learn everything you can. Also before that find some of the good saddle fitting websites.
Saddle fit is just as important as proper farrier work for keeping a horse sound and useful.
It is possible your saddle is a good fit that just needs a little tweaking or it is possible it is a bad fit overall.
The shim saddle pads people are suggesting all.cost in the $100 plus range so it is money well.spent to get a saddle fitter involved now before you accumulate a library of pads and shims that you don’t need and that don’t fix this problem.
Good advice at the start always pays off IME!