Not long ago I took some lessons from a well known and well respected player, and he helped me clean up my technique quite a bit. My one continuing problem seems to be excessive tension in my left thumb. I've accounted for all the culprits I can think of -- hand position, needing to stretch, playing position/strap height, etc. My instructor told me to "just relax." Good advice, but I just can't seem to do it. Anyway, this problem manifests itself as pain/fatigue felt in the crotch between my left thumb and first finger, when playing repetitively in one position. Any suggestions?