Just a thought since I ride and breed 1/2 Arabs.
Are you REALLY sure she’s narrow? Or is she wide/flat on top, with a small body?
That second, for me, is the toughest combination to ride. Hips have to be SUPER open, but then there’s nothing there from mid-thigh down. Unlike the ‘pear shaped’ wide horse who can be the equine version of a couch.
My stallion is little. But takes custom wide trees (a Duett 40 is not quite wide enough) I rode him treeless in a Fhoenix for a couple of years and he was blissfully happy. Finally he bulked up again–beginning 3rd level dressage work–and it was just TOO much for my hips and the suckingsaddlesearch began again. When he first came my balance on him was ATROCIOUS. I could not ride him bareback for the life of me, even at a walk. And I was riding 4-5 horses a day, 4x/week… all shapes and sizes… it was the combo of his extravagant movement, shape and size. He is still not an easy shape to ride, but with strength and sit behind… and almost 10 years of work at it, he is now at least ‘secure’ feeling. The wrong saddle on him though, and I still feel like I’ve never been on a horse before… I think other shapes you can get away with things more.
If the horse is quite narrow, padding the UPPER thighs (think the big thigh blocks on dressage saddles as high up as it can go) can help make you feel more secure. If the horse is wide/flat on top, it’s all about opening the hips. And when you are nervous or excited (as in your CTR) you tend to tighten… opening the hips takes tremendous time and effort, the tendons involved are some of the biggest and strongest in the body. Retraining them is a lifelong task. The right shape and placement of thigh blocks can help. (they can be added, or velcro can be added fairly inexpensively) the exact placement is going to vary by individual, but higher is usually better for stability and opening the hips.