It looks to me like she is suffering from Lordosis, which is a degenerative issue and causes swayback. I am sure that a vet could say for sure but if it is that, then no amount of exercises or belly lifts will help. Supposedly, horses suffering from Lordosis are not in any pain and can actually be ridden (though I wouldn't try it, especially with a saddle) without hurting them. At this point, it isn't something that can be fixed but she appears to be healthy and happy so the remainder of her life might be best spent as a companion horse. She is a very pretty little girl and I love her ears. I adore mustangs.