Both. When I look at your avatar photo, the pelvis looks quite vertical and the upper body is over the pelvis quite nicely. I think that perhaps, as you get going faster, you grip up with your calves, which causes you to have toes down and heels UP and you lean back too much.
So, look at your avatar and think that position , even at a good quick canter/gallop. Relax and let your legs hang down. Do I know this feeling? You bet I do! I know how it is, finding yourself gripping on. Do you lose your stirrup at canter? I used all the time, especially the right one (the stronger leg, which was doing the bulk of the gripping up)
Start riding at a slow canter , find a spot where the horse has settled into his gait and it's pretty rythmic, and RIDE him and relax and really think of your legs goind down his sides like the two halves of a clothespin (maybe an old image)
Anyhoo, just keep that image in your avatar in your brain and BE that lovely rider, even at a gallop.
Easier said than done, I know, but one works toward those things. Savor each moment of success and each one will become close to the next one, until they are nearly nonstop.