She is perfect where groundwork is concerned so I see that as a waste of time for you with this horse.
You have to learn to be a very relaxed rider - I had a lifetime of fizzy gymkhana ponies so to me she was no problem, I love her the way she is, you either like this sort or they terrify you and if they terrify you you will send them into a panic as they need cool and calm
I have long perfected the 'sack of spuds (Potatos) tied up horrible' style just for these sort of horses.
If you use more than a hint of leg it will be off but you need to keep them into the bridle on a light hand - any hint of a hard pull and that forward motion has a nasty habit of turning into an upwards one.
One point - is this horse 'hot' or is it fractious?
You might want the owner to look at possible pain issues - like in her back or maybe even ulcers.