OP
I get your frustration.
I dont think at this point that this is the trainer for you.
My suggestion is to do some research in your area and find a dressage trainer who starts young horses or an event trainer who can teach BN .
You may be able to take some longe lessons and can work on your position issues on a dependable steady horse so that you concentrate on getting an independent secure seat
This will build your confidence and security.
I do believe it wont take you long to do this.
A few sessions ought to do it.
I think you might even come to enjoy taking a few dressage lessons on your horse . Relaxation is one of the basic building blocks of dressage and I think it would benefit both you and your horse.
I understand that it may be difficult to find a dressage instructor so I also suggest looking into finding an eventing instructor as they teach lower level dressage as well.
And you dont have to buy a dressage saddle either
I ve known plenty of riders who started taking dressage lessons in jumping saddles and did just fine.
I am not suggesting that you take dressage lessons forever. Just until you regain your confidence.
Hope this helps.
Good luck.