I have gone through this, I was looking to see what the responses were!
My mare is half your pretty horses size, but its relative. She is not arab, but has an "arab back", ya can just roll right off of!
Sooo, I got her a saddle (western) w an arab tree. Then, I put a roper cinch on it, my theory being there is more surface area contact. And, I always use a breast strap...which is a pain, but does help. And, while all these measures have helped, it will never be like "normal". So, I am just extra careful about keeping my weight balanced.