OK...my mare canters much better on trails than in the arena. That may be because it is easier to canter straight, or it may be that she simply understands what she is doing better when she isn't in a near continuous turn.
When we canter in the arena: I hurt my lower back shortly after I took up riding 4 years ago and it is still pretty stiff on my right side. I find it very hard to sit well back in the saddle and flow with the horse because my back doesn't move the way it needs to. Also, both of my horses used to leap into a canter as if there was an invisible fence to jump at the start of a canter. I'm one of those who ended up learning to canter on horses that didn't know how to canter with a rider. I tended to lean well forward because otherwise it felt like I was going to come out of the saddle.
So I was starting the canter with my body too far forward, and they were following my body onto their front legs. I think they were also trying to escape my butt slamming the saddle into their back by hollowing out their backs and trying to put everything on the front.
I started using some *******ized version of a half seat where my butt was barely touching the saddle. Being a more forward position, it kept me from feeling like I was going to fall behind and thus kept me from tensing up. It also freed up the saddle to move with the horse's back, and therefor helped them to be more relaxed. And with practice, I'm getting better at relaxing back a bit and slowly entering the saddle, although I still don't ride a canter with all my weight in the saddle.
Here is a thread I bookmarked because it really helped me. And BTW, I use an Aussie-style saddle with my mare and a western one with my gelding, so I'm not an English rider: https://www.horseforum.com/english-ri...f-seat-120340/
All this seems to be helping my horses to relax, and when they relax they can also turn better at a canter.
It is hard to describe it in words. I also fully understand that good teachers aren't exactly easy to find in many parts of America. I tried getting instruction in cantering from two instructors who were very helpful to me in some areas, but not for teaching someone to canter. One taught entering the canter by pulling the horse's nose to the outside of the turn, which I think unbalances them and makes it harder.
I have no idea if any of that applies to you. I own three horses and can only say what has or has not worked for me.