He is overflowing with personality, but is a very kind horse. I would not presume to offer medical advice, but my suggestion is always to consult your vet. Not every horse will respond in the same way to any treatment, and of course I have never seen yours nor am I a vet. If you are just beginning dressage, might it be partly that she is using her body in a way unfamiliar to her? Joe Joe (the horse one, not the me one) had some issues with that, but it was more that he prefers to carry his head stretched out (the better to win the race we are NOT running) than stiffness in his joints. He did grow accustomed, so maybe that time is all your girl needs?
It is never the horse's fault