thanks tiny. It's really frustrating and she is starting to come round nice and light in these lateral flexions compared to when I first started them when she needed alot of support and couldn't keep her feet still enough to bend her neck even slightly round.
It's sounds like you are approaching it the right way. Suppling through ground exercises is better than from her back. If a horse cannot do it on the ground correctly then ridden will never be better.
After years conforming to certain stresses and demands it will take time. Bones can even remodel. Little and often is always the best way, and never force any flexion or stretch, always let the horse find it, just as if I was asking you to touch your toes. I wouldn't press down on your back I would just keep asking you to go as far as you can without hurting yourself.
Always remember it will start with the TMJ, work through the neck and then the ribcage, through to the hind ultimately. Also remember to work on straightness exercises too , and always work both sides the same even if you are concerned with just one side.
Some useful exercises include poles. Place some poles in a fan shape, so at one end they are all touching and at the other they are apart if that makes sense? Then lead the horse and ask her to walk around that curve over the poles. You can even make it harder by raising one end of the poles, and even alternating them.
Another great exercise is to stack lots of poles across each other like Kerplunk (British people will know Kerplunk). Always make sure one end of a pole is on the grounds they are stable. Then ask the horse to find her way across. This means she has to choose her own steps. It encourages, trust, concentration, balance and strengthens tendons and ligaments that get neglected.
Another one is to make a labyrinth of poles, imagine an S shaped passage to start with. Leading the horse through makes them bend around the corners, but also gives them a focus and allows you to release any pressure on their heads so they can flex naturally. As they also have a reason to bend it provides more interest and purpose for them and settles them a lot.
Hope this helps.