I was going through the same thing with my new Mountain gelding! None of my three saddles fit him well at all. Couldn't find a good fitting saddle that wasn't sticking out way over his loins because of his super short back, wasn't too narrow for his mutton withers, or wasn't too big to hinder his freedom of movement/gait. It was all new to me as nearly all of my prior horse experience is in vastly different breeds.
Lucky for me I prefer english or Aussie saddles (ie: smaller than those used western behemoths in my price range) not so lucky as I seem to be the only one for 200 miles around here that does. So, no saddles to found anywhere near me. How is a girl to buy something online that she has never seen in person, let alone sat in??
Well this ensued lengthy web research sessions to learn better how mountain horses need to be fitted, how to take wither tracings, measurements, what are wither angles, gullet sizes, tree sizes, tree angles, molds of the back, sweat stains, and which way the wind is blowing during each of these measuring things that need done.
Then I took all of this, plus the fact that any saddle sits differently on different breeds (and horses) and narrowed down my saddle type choices to ohh.... about 20 or 30 different saddles.
Through all my research (especially on the Aussie saddles) I kept reading how good the folks are at DownUnderWeb. Having checked their website numerous time I knew I could get a western, english, and of course an Aussie saddle there without totally draining my wallet. And their videos are very helpful.
So, I finally called DownUnderWeb and talked to a saddle fitter over the phone. About 30 or so seconds into the conversation after I told him my fitting problems he asked what breed my horse was and as soon as I told him he said "ahh, I know exactly what you mean." He assured me that he could help and after I described my horse and his measurements, plus my size and height (to make sure I was thinking correctly on an Aussie size) we went through my list of interested saddles and he told me what would and would not work on my horse. He also told me (based on my riding style preferences we talked about) which saddles I would probably think were more comfy than the others.
He never tried to sell me any particular saddle (as in... one of the really expensive ones) and really listened to what I wanted. I also asked a bazillion questions about the Aussie saddles because I just want to know everything there is to know about them and he patiently talked to me for at least an hour.
If you have an idea of what you want and if they have anything remotely close to it on their website... call them. If nothing else you will gain a better understanding of what your horse needs and how to find it, wherever you choose to look for it.
While I'm waiting for my non-custom, non-budget breaking new Aussie saddle to arrive at least I have way more confidence to be able to shop for used western and dressage saddles anywhere I want to now, so long as I can get answers to my specific measurment questions myself or from from the seller.
I didn't need, nor want a custom saddle for my gelding yet either as I would like to save that for the expensive one I plan to get a few years down the road, plus I'm still recovering from how much he cost!
Now I'm not going to say I got all of this from just one phone call to the saddle place I mentioned above. It was all the research, measurements, local western tack store visits (for them to say "Yup, you need QH bars.") and the phone call that helped me put it all together.
Or perhaps you could just skip all my exercises in research and frustrations and go straight to the phone call.
It might really help get you pointed in the right direction.
downunderweb dot com (not affiliated in any way... just happy for their help.)