Take this with a huge steaming cup of FWIW, but it helped me with cantering:
Riding the canter in half seat
Riding the canter correctly and well in a full seat is difficult, and many more riders do it badly than do it well. As Allison stated above, it requires a degree of abdominal fitness, as well as correct position, relaxation and a good understanding of gait mechanics and how the horse's back moves. That's out of reach for a lot of recreational riders. I would much rather see an elementary or intermediate rider cantering in half seat, allowing the horse to move freely, than someone attempting and failing a full following seat and punishing the horse's back in the process.
There is nothing inherently insecure about riding the canter in half-seat or two point as long as the rider is in balance.
That is from an English riding thread, but most of my riding until recently was done in either an English or an Australian saddle, and I preferred riding with a forward seat. I also have a stiff lower back from an injury shortly after I took up riding. And I found it very hard to truly follow the motion of the horse's back so that I was cantering with the horse instead of "bouncing the canter".
So I started entering a canter in a half seat, and riding it that way. My 1/2 seat slowly progressed, depending on the day and my ability, to a 5/8 seat, a 3/4 seat, etc. In truth, even now in a western saddle, I probably enter the canter in a 3/4 seat, so to speak...and sometimes stay there.
For western riding, that is utterly untraditional. But it works for me, and it saves my horse's back. Some days, when my lower back is OK, I even ride the canter in a full seat. But I honestly think many people do not ride a full, deep seat canter well enough to allow the horse to use its back to the best of its ability.
No REAL western instructor would make that recommendation, but it worked for us.
A big part of what has attracted me to western riding is that it generally allows a rider and horse to do what they need to do to get the job done. With my background and my limitations, that was a good way to ease myself into a canter, particularly since my mare was trying to figure it all out at the same time I was! I didn't want her to dislike cantering, so I did what I needed to canter without hurting her and we've worked our way slowly forward ever since.
Also, on the same thread, Allison posted this good video - English riding, but so what?