Something to try. Try fretting without using the thumb at all.
With a minimum of practice you can play many pieces without the thumb. You come to find:
- the fingers assume a better position,
- you the fingers need to assert much less pressure than you thought,
- you feel different muscles in use,
- you need almost no pressure on the thumb
-- it's just there for stability,
- the natural position for the thumb is generally at the centroid-of-pressure asserted by the fingers
- each time the fingers change position the thumb may need to move too
- it's easy to get into the habit of using the leaving the thumb stationary, as a navigation aid, when the fingers change position thus throwing the left-hand into a poor position.
For me, any time I'm having trouble in the left-hand, the first thing I do is take the thumb away and see what the fingers want to do without it; solves lots of issues.
"An amateur is he who takes up the study of an instrument as a relaxation from his serious occupations." -- Sor