Originally Posted by Mercedes
It sounds as if in your attempt to 'hold' your legs, you're using the wrong muscles, which in turn is creating the ankle issue.
You don't actually 'hold' your leg...more accurately it should fall from the hip and drape along the horse's sides in a passive manner until such time as you need to cue the horse.
This is correct, but you still need muscle tone to do this, which comes from keeping a still leg. Those of us who have ridden dressage long enough have enough muscle tone to drape our leg and keep a still ankle, it is when you don't have the tone that you get wiggly ankles.
Like ridergirl said, you have to gradually lengthen your stirrups to a comfortable position in order to build up muscle to hold your leg still.
If you still feel your ankle is wobbly then make sure you are riding in regular fillis irons, and look at purchasing a pair of stiff dress boots if you are really serious about doing dressage.
Otherwise, just keep riding and your position will improve if you are conscious of it.