I would work on lunging her daily and take her in-hand on "walks" until she loses a fair amount of weight. I would also evaluate her diet to make sure it's as calorie reduced as possible while still allowing for proper gut health/function and nutrition. Once she's nearly what you want weight-wise, call up Duett and get a demo saddle to try out for you both.
I was able to pull weight off my mare over the last 3 months by restricting her hay to only what she needed (2.5% of her target weight in bermuda hay). Her "feed" consisted of a vitamin/min supplement and 1/2 cup of flax meal mixed with 1 lb of chopped alfalfa hay and 1/2 cup of apple cider vinegar. Her pasture is overgrazed, so she only had a little grass to nibble on to keep her busy. I'm happy to say she's almost ribby! Lol I prefer my horses "lean" with good muscle. She's now back on free choice hay (round bales) and riding 4 hours a week, but with no increase in her "feed."