Cushings can be managed. One of the horses at our farm has it, and is still quite frisky at age 36.
As for the rest, I recommend a good trainer – one who will also be willing to get on the horse, as well as tell you things. If you want to look still, sit still. When I was young (many eons ago) we had to learn to ride without reins or stirrups. Not saying you should drop your reins, but you could cross your irons, and rely on your seat and legs to keep you where you belong.
It is never the horse's fault