I so feel for you, I to am struggling to start really cantering again, I used to be fine, and now I'm not, go figure.
The most repeated word my trainer yells at me...........BREATHE, every time I get tense I forget to breathe, which makes me more tense, and if I'm tense I can't sit in the horse, I end up perched ON the horse and bouncing...sound familiar?
I have asked at my next lesson if we can either go into the round pen or if she will lunge me, in that way I don't have to worry about controlling my horse only about the actual movement and feeling of the canter. It also means that I can have a neck strap and hang on, so I can concentrate on that wonderful feeling.
I KNOW that cantering is so much more comfortable than trotting, I know that there is no where for G Man to go, if we are in the arena or round pen, he can't run away, so my fears are really unfounded.
Being so often a nervous rider I can share that others have said works...
BREATHE........breathe out lots, your body will breathe in on it's own, it kind of wants to stop you suffocating, so concentrate on out, before asking for anything from your horse, breathe out.
SINGING, is a great idea, I don't do it so often in lessons but when I'm riding my horse at home, I sing out loud, helps me breathe and relax.
POSITIVE THOUGHTS, I know I should do this in all areas of my life, but I can't, I am though getting much much better at doing it when I ride, first the thought, then the action. I 'see' myself doing perfect canter departures, I 'see' myself doing perfect downward transitions, I know I cam do them, and I can see them in my head, that is the model I'm aiming at in practice.
Right you and me both, I have a show in two weeks, it will be my last walk trot test with Mr G, after that we move up to training level, and I WILL have to canter. So lunge line here I come