I *love* Master
saddles. My saddle fitter had a Master and Marvel model that she let me try and I was actually mad at her for a little while after because it was SO much more comfortable than anything else she showed me, but even used they're generally out of the price range I had given her. I ended up saving up a little more money and got their other dressage model, Merit (it took me a while to find one anyway, so it worked out; they're a little hard to find, but definitely not impossible)
All of their saddles have a wither gusset which helps to fill in that space behind the withers so it doesn't sink down on the withers like the problem you've been having. I'm also a petite rider (5'2" with a short inseam, even for my height) and these saddles work really well for me; I didn't realize how much I was fighting my old saddle to stay in a good position until I tried these saddles.
I liked the Marvel model the most; the Merit that I ended up buying is almost
as awesome. The Master that my saddle fitter had was also nice, but when I found one of those for sale and tried it out it didn't seem quite as comfortable; it was a slightly bigger seat size, so that might have been the reason.
These saddles get sold as both "Mike Corcoran Saddlery" and "Master Saddlery D. Boot and L. Bachelor" The models are built the same, but have different plates on them.
You can usually find a used one in very good condition around $2000; they're over $4000 new. The Marvel my saddle fitter showed me that I initially fell in love with was a custom made saddle with short flaps; I'm not sure if the one I ended up with also has custom short flaps or not but I suspect it might; it fits my leg length well.