Use the end of your brush, the wooden part and run it along her seam line underneath. When you get near the belly button she should lift her back. If she does, with a flat hand, tap the middle of her back a couple of times then lift her again.
With the right work and feed it will improve. My old mare had a sway back at 20 and worked for the next 9 years it did improve. But saddling needs to be sympathetic and checked regularly. Posted via Mobile Device