Just be relaxed and try to follow the horse's moves. Practice that for a long time & don't be scared. I hade the same issues, but then it was okay😉
Hello Macy !!
The key is to just hula hoop in the saddle
The key to seat well while cantering is to be relaxed; (Easier to say than to do!)
Usually when you don't follow, it means that you have tension in your pelvis and your knees, you need to make a lot of work without stirrups. You need to be seated on the fat of your butt, keep your shoulders behind, your head up and use a lot your abs to really keep straight your back.
Your pelvis follows the movement by going from behind to the front of the saddle.
You can also try to think about your position by riding while closing your eyes to focus on your position and "feel" the movements of your horse.
You can also extend your leather and try a new saddle to see if it fits you more.
I hope i could help you, good luck :)
Always try to maintain your grip by using your thighs and knees. And try to lean a little backwards, which will help you keep your seat during the canter