Whenever I see the shows with the mostly western trainers on RFDTV, the first thing they work on for suppling is to bend the horse around the inside leg. It's MUCH more extreme than the dressage version of flexion where you just flex the inside rein enough to see the inside nostil and eye. With the western guys, they pretty much try to get the horse's nose to touch his own belly. At the halt, drop the outside rein and then gently take contact on the inside rein and bring it back toward your hip. If the horse braces against the contact, bump her with your inside leg the get the whole body into the bend. As soon as she gives to the bit, release the contact. Repeat on the other side. Do it to both sides a few times until you get a response that your satisfied with. She'll probably turn around in circles before coming to a halt and giving to the contact. It's just her way of saying she's stiff. When my horse feels extra stiff, I do that exercise at the walk and apply my inside leg in a pulsing way to keep him moving. We bend around in little circles one way, straighten for a few strides, and then go the other way. It might work for you. Can't hurt to try.
If she supples to that, move onto shallow serpentines to try to keep her supple while moving without such drastic bending as the above exercise.
good luck. Stiff horses are a challenge. It's like riding a piece of plywood.