My gelding is 27 and his back is also hollow/sinking with higher withers. I don’t think there is anything we can do—I have a thick saddle pad so the saddle doesn’t cause problems but I think the topline is gone. I try to keep his weight even so that if he lays down he is able to get back up without a lot of added weight. My horse has never been “weighty” but of slender build. He looks good except for his back. Pasture 24/7. You could try some suppling and collection exercises to maybe help your horse round her back and collect herself but if it’s sunk, it’s sunk. Age, there is nothing we can do about it.