Are you doing hunters or event/jumper? The canter is addressed totally differently between the two types of disciplines.
There is a difference between canter position and galloping position. In event/jumper, you do sit the canter....but when jumping at speed, you will revert to the galloping position where your seat is out of the saddle. In eventing, you would never go XC sitting the canter, except MAYBE a couple of strides before the jumps at the lower levels. Sitting would put too much strain on the horse and cause him to tire in his back. Up and out of the saddle offers the least resistance on the back and allows the horse less restriction. The photo you showed was an XC photo.
Hunters do not sit deeply at the canter. Their desire is to move as little as possible on the horse to make it appear like the ride is effortless. The tighter more arched back tends to lock the seat, making it difficult to sit deeply.
So, when you changed coaches, did you change disciplines?