Absolutely beautiful horse
I totally understand the problem, I have 4 saddles right now for sale that don't fit my Rocky Mountain horse. One was even a custom made Rider's Choice, guess what? Still doesn't fit!
I found that a very short saddle with a round skirt fits best, not perfect, but good. It is a Fabtron, but has a horn I don't like. The Endurance style they sell has a tree that doesn't fit my horse. So I called them and discovered they will make me a saddle to fit my horse. I haven't done it yet, because I am planning to demo a treeless first.
So, one option I would suggest is e-mailing Fabtron (maybe other companies will do this too) and have them send you some trees to try, then they will build you a saddle on that tree.
Second option - Since a Dressage saddle fits well, what about trying an Aussie saddle? You could get one with a horn & have more of the security you are looking for with the fit of the saddle you are using.
Also, instead of buying saddles, only deal with companies that have demo options, at least you will not be out as much money. Once you discover one that fits, look for a used one to save $$
Good luck to you!