CY - there are many good replies with good information to this question. I am curious about one point: does the tip joint of your thumbs hyperextend? E.g. are you able to form an "L" shape with the last joint extending away from the palm?
I ask about this as my thumbs are quite double-jointed as they say. This creates challenges for both hands in different ways, but it could certainly contribute a great deal to excess pressure in the left thumb as it becomes very difficult to use the tip pad of the thumb for support and remain relatively relaxed in the proximal thumb joint and ultimately the wrist.
If you don't posses this ....um physical feature, then nevermind