It's pretty normal to have that experience of uphill cantering being more comfortable. I feel the same way. When I canter out on the trail on the flat, I have to think a bit about really sinking down into my horse , INTO his body, imagining that my butt is 6 inches inside him, and letting my feel fall down into my stirrups. I have to repeatedly tell myself this and redo them because I will tense up , so I just keep reminding and redoing. So, for practicing purposes, do more canter work on the flat.
I don't canter downhill all that much, period. Too uncomfortable for horse and rider.