Maybe you could try doing a two-point on the flat: walk, trot, and canter. It really helps because if your leg is out in front of you, you tip back into the saddle and if it is in back, you tip forwards. When you're doing that you can work on using a proper crest release- it's easier on the flat than jumping. Have you done any trot poles with her? It might help a little. You have a really flat back in all of the pictures.