It's not entirely about lowering their head and yielding to the pressure. A horse will come into an outline when the whole of their body is engaged. The horse needs to engage it's hindquarters and work through the outline. Side reins, chambons and the like force the head and hollow the back. A gadget like a pessoa will engage the hindquarters, the back and then you will begin to achieve the long and low.
Perhaps long lining her might work, they may be to similar to side reins, but its worth a try
Really set her back on her hind quarters and drive the power from the backend, she will start to lower her head after a couple laps around the ring :)
Maybe try on the ground working with her to give. Also can do excersies for bending and counter bending just to use those muscles that he/she will need to give and collect.