This worked well for me, although it might be tough in a lot of western saddles...
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.
At a canter, the loins of the horse go up & down a lot. The withers barely move. In a half-seat, you put a lot of weight into the stirrups, which hang nearer the withers than the loin. You are leaning forward, so your weight is off your rump and on your thighs. Your rump is barely touching the saddle. If you are leaning forward the correct amount, you could transition to standing up in your stirrups without needing to adjust your upper body. It is easy to practice it at an easy trot.
When I tried that, I stopped bouncing on my horse's loins, which made both of us happier about cantering. From that position, I can slowly settle into the seat as I feel the rhythm of the horse. Or I can stay light in the saddle and keep going. But with time, settling deeper in the saddle only when I felt the rhythm, I got used to the horse's motion and moving with them.
If you bounce while learning, you also teach your horse that cantering is painful, and you teach your horse to lock its back to protect himself from the bouncing - and the horse's stiff back makes it much harder to ride. It is like sitting the trot. An easy, relaxed jog is easy to sit. A nervous, stiff-backed hard trot is painful to sit. The same thing happens in a canter. A relaxed, easy canter - a lope - feels good to sit for both horse and rider. A nervous, stiff-backed fast canter is like sitting on a jackhammer.
I hurt my back a few months after I started riding at 50, so just relaxing with a flexible lower back wasn't an option for me. Getting my horse to relax, collect some and be happy in a canter - ie, to lope well - makes a big difference in what I can do. For me, learning with horses who were not experienced in cantering themselves, starting in a 2-point, settling to a half-seat, and then moving deeper into the saddle at MY pace has helped a lot. Before that, I looked like I was flying in formation with my horse instead of riding him.
If that doesn't help, just ignore it. My youngest daughter and my daughter-in-law are both far more flexible than I am, and they pretty much just sat & relaxed until it felt right. And of course, being about 75 lbs lighter than me meant they weren't punishing the horse the way I was...