It is lovely to see a very tidy rider with a solid lower leg, none of this wobbling, swinging lower leg that is difficult to correct, well done :)
I do agree with what some others have said, that you are more of a passenger than a driver at the moment, and I think that your position is good enough now to be able to take a step up and 'take the handle bars' so to speak.
My biggest concern is your hands. Please refer to my response to this thread: Anything i need to improve on while riding??
for my 'solution' to problems with hands, I believe it will do wonders for you :)
Once you have your hands sorted out, I think you will find that your balance improve ten fold and your lower leg will start to come back beneath your centre of gravity. Holding your hands down at your crutch makes it much harder for a rider to maintain a strong core strength to remain upright and effective in the seat. You will find that once your hands are lifted forward and higher, you will become more effective in your ability to use your seat, and will start to become a driver more than a passenger.
You have a slight chair seat, but your lower leg is so stable that this is not a huge problem. Again refer to my post in the linked thread, as I think you are gripping a little with the knee so my advice on the other thread regarding the upper leg will apply to you as well. This is more so evident in the canter than the trot. I feel that the problem you have of rocking in the canter and being a passenger, is because of the gripping of the upper leg. When the upper legs grips, the hip locks and you will start absorbing the horse's movement with your upper body rather than through your pelvis and core muscles, which induces the 'rocking' motion in canter. If you can unlock your legs and pelvis, your weight will naturally come back to your seat bones and allow the pelvis to roll with the canter, absorbing the motion and allowing the upper body to remain still.
Another tip for a chair seat, is instead of thinking of pushing your heels down, which subconsciously makes a rider not only push them down, but forward as well, creating a chair seat, instead imagine that you are kneeling down. Kneel your weight into your heels, if you kneel with your lower leg in front of your centre of gravity, you will tip backwards ;)
Again, so you don't think I'm being a big meanie and saying it's all bad - I do quite a lot of coaching to supplement my income, so I am being VERY picky! You are a lovely, tidy rider with a good foundation for being an effective and talented rider in the future. Best of luck.