I've been playing for a couple of months now, and I have a pretty good teacher, but I'm running into a pain in my left thumb quite a bit, makes it almost unbearable to play a song more than 3-4 minutes. I drink plenty of water and stretch before I play, which makes it a little better, but it's still just painful to play for extended periods and it's hurting my ability to make smooth transitions. I think I have an idea of where the problem is coming from: my thumb moves all over the place on the neck. I've been advised to try and keep the thumb in the middle pointed toward the nut at all times, but I have big hands and I'm playing on a J bass neck, and it's really hard to keep my hand balanced, especially when playing fast. It also puts a lot of pressure on my wrist. I get the most pain going up to the G string quickly, I notice my thumb is almost perpendicular to the neck and touching the side of the fretboard.