Here's a good exercise:
On a large circle, establish a forward, rhythmic, connected medium trot. At a designated point, transition to walk and make sure that the walk is marching, pushing, powerful for 20 steps or so, then step up to canter (no trotting in the transition), then after a circle at canter, back down to medium trot. Repeat. Do both directions. You can also change it up by placing poles at random locations on your large circle.
The above posts are great suggestions. I would also back your horse. You'd be surprised at how well simply backing and then moving off will develop the top line. The key is not to let them become hallow in the back and ewe necked when they are backing, they should stay through in the contact or if you're doing it with a halter, use a treat to keep them from going above the vertical.
You do not need the designer training "system" to fix anything here because it won't fix what is missing. Plain old regular and consistent riding over a period of time will do that.
Just riding out of the ring over uneven ground and varying footing will help her, even without hills.
Tying her head back with one of these things will do nothing at all to strengthen her back end and her conformation may not support that anyway so you will end up doing more harm then good and get her not wanting to come forward.
She has never had a good, consistent regular program. Just give her that, tend to the health issues and give it time.
When opportunity knocks it's wearing overalls and looks like work.