I recognise all this. And the answer depends a bit on how long you've been riding? And how long you've been riding canter.
There are two elements to this problem. Canter is only one of them. The other is the way we good students repond to instructions from teachers.
You were told to ride with your seat.
You applied the teaching 100%, to be perfect. You rode with your seat.
Then you were told your seat was "too loud".
One of the great causes of failure in my riding lessons is over achievement. You may need to alter very litte? Be moderate in applying alterations to your riding. Probably all you did was go to far, but in the right direction, if you see what I mean.
Now to the question of seat in canter. There are people on NR who "use" their seat. That is not how I am taught. But I am taught to SIT on my seat bones - in all gaits and to allow them to move with the horse.
If you are a dancer it should be easy for you to do this. In canter one can transform the situation just by breathing deep and slow, just relaxing your seat and letting your legs hand down. You wont bump any more so you wont need to grip with your knees. But it is a vicious circle, if you grip again, you will start to bump again.
As you stop gripping with your knees, it puts more weight on your seat bones, lowers your centre of gravity, allows you body to be moved by the horse and lets the horse go more easily. The movement originates in the horse and not in you.
When your teacher says your seat is too loud she presumably means you are initiating the canter movement instead of just allowing it.
You might need a bit of practice in canter out hacking or somewhere where you have time to think quietly about the breathing and rhythm without having to worry about corners or keeping the horse moving.
Now the irony is that, because it is easy for a horse to carry you when you are not gripping, most horses will go faster for you (learn how to do downward transitions from canter or how to slow canter too).
Some people may then allege you are driving the horse with your seat. I have had that said to me. But I am not. The way I am taught to ride is to do next to nothing and allow the horse to move.
If you are a dancer who ever dances with a partner you will probably find you have a gift for this.
One other thing I did which relates to learning dance steps e.g. in a walz was to work out the movement to expect under my seat bones in a transiton to canter? I did this in a lunge lesson. No one told me to, I just did it. You will feel your outside seat bone dip as the horse takes that back leg off the ground and then the next moment that same seat bone is thrust upwards as the horse pushes off.
After a couple of times your body absorbs the information (of what your dance partner is doing) so that even the first step of canter seems smooth.