Proper left thumb position

One of the words of advice heard repeatedly is to not grip the neck too tightly. My thumb ends up hurting after a couple of minutes which indicates to me that I'm gripping it too tightly and I've tried to consciously ease the pressure on my thumb, but I keep getting to a place where it hurts. It's only been a week and a half so my question is, is it like the thing with calluses where trying to find that proper pressure is just a matter of time and practice or is it something that needs to be paid attention to constantly?

Your thumb muscle will develop just as your calluses will. You are correct, it is a matter of time and practice. In the beginning you might need to pay attention to that aspect of your hand position constantly. But over time it will become second nature. Of course, don't injur yourself! If it's really hurting, take a break, do some stretches, and come back at another time.