I've always struggled sitting the canter, not like I'm unbalanced but actually having my physical butt stay in the saddle and yet also be effective.
For a long time I guess I did a weird whiplash thing with my back that all my instructors would cringe at, yelling "Stop doing THAT!" I really don't know what "that" was, but somehow I stopped whatever "that" was. One told me if her back ever moved like mine, she'd be at the hospital...
So now I guess I kind of "float" or hover, not really perching, knees aren't pinching. This is especially apparent on horses with a lot of lift in the canter. Previous horse was a QH disguised as a TB, so it was flatter and I could sit it or maybe just got away with whatever I'm doing. Sometimes I can tell I'm doing it and make a concentrated effort to sit deeper, sometimes I think I AM sitting, then I get a "SIT down!" from my trainer... It sounds like a dog obedience session during my lessons at times.
I do have lower back issues, lots of tension, disc compression and some nerve pain. Been through the ringer with Dr.s, PT, chiro, injections and nothing has been able to help. I think this may be a large part of it, that I brace a lot with my back while riding or oddly enough, tend to over-arch my back(this is usually my fall back position that I'm most comfortable in).
I'm better with visuals. For a while, just working on pushing my hips more forward would be enough similar to a yoga "pelvic loop". Right now, I keep telling myself to "melt into" the saddle to try to relax and loosen. However, I'm starting to see with me "melting," I now brace somewhat through my legs and overflex my ankles. This happens in a multitude of saddles, so I'm pretty sure it's me that's the issue.
I've had similar issues to yours before. I have to remind myself to sit up, back, and on my butt. When cantering I let the tension go in my legs and back. My leg is still there on the horses side and effective but it is not braced. I basically have to imagine myself flowing through the horse. Legs long and quiet. My butt is in full contact with the saddle. My upper body is tall. If I feel myself doing otherwise I just take a deep breathe/brief moment of regrouping (while still cantering), exhale, sit up, back, and melt into the saddle.
If you have an issue with arching your back too much, try tilting your pelvis upward and engaging your abs. That should take some tension off of your back as well, if your core is properly engaged.
Have you done any work sans stirrups? Either by yourself on the horse, or with the horse on a lunge line. That may help you focus on keeping a better seat, because quite frankly, you have to