The thing that I see immediately is that you are coming way up out of the saddle, then your bottom is slapping the saddle and you come back up. Why??? because you need to bring your shoulders back,,back to AT LEAST in line with your hips. Stop the video at 0:36 and you'll see how high out of the saddle you are, and how much in front of your hips the shoulders are.This is a very unsafe position to be in and will make it much more likely for you to come off if anything at all happens. It's not 2 point and it's not a sitting canter position either.
Is that an instructor in the center ????
There's another thread on here where TxHorseman has posted a very detailed description of a nice balanced seat...maybe you can find that ?
Imagine if you were holding a thin pole or yard stick and it should go from the back of your heel, thru the center of your hip (seat area--centered from front to back--does that make sense?), to the center of your shoulder (middle of the top of your arm). Hope that makes a nice picture for you.
For the canter, the shoulders back even a bit further can help , your butt needs to be deep in the saddle and your body/butt should move forward and back with the horse's movement --you dont do any moving--just go with the horse's canter movement. NOT up and down AT ALL. Your shoulders should be pretty still ,,although it takes a trained eye to realize the riders seat is all that's moving,,,,not the shoulders.
Also, for what its worth, slapping the saddle with your seat for very long at all can possibly give your horse a sore back, and make him not care for cantering.
If that is an instructor in the center and she/he has not tried to correct your position, you need a new instructor! for your safety and the horse's comfort.
Looking at the very first frame in the video, your shoulders are nearly back far enough, it would help you, imho to soften your back (not be so concave) and it would really help you and help the horse balance if you can ride with a much looser rein. This is part of what's pulling you forward-the short rein...imho. But then, I ride on a very loose rein , so I could sure be wrong about that.
He looks tight in the neck to me,and that might be making his stride choppy feeling, though it doesnt look too bad to me. He did pick up the canter pretty nicely for you ! I'm not sure if he's so much counter bent, and looking out, would need to look again to say.
My instructor had me ask for canter, then canter only a few strides. Get used to the transition..that first leap into the canter , esp. from trot, is the hardest part of riding the canter for a lot of people. Then once I could get a nice transition AND keep my butt in the saddle...those darned forward shoulders!!...I got to increase the distance. It is a slow , but safe and confidence building process...and I'm still working on it myself, so I know your struggles first hand. The last few times I asked for canter-not in a lesson--I was doing exactly what you are doing....coming out of the saddle and slapping the seat. *sigh* but I didnt keep going around the arena, I trotted or halted after just a few strides/slaps.
I hope this is helpful to you. You will get it !! stay safe and have fun!