A couple of ideas. On a well set-up guitar, you don't actually need to use your thumb at all - by using your whole arm (l.h.) to pull in, and your right arm to pull in, in the opposite direction (your body is the fulcrum), there's enough leverage and strength to take your thumb off. In fact, I'd say the majority of players squeeze too hard.
Using your thumb - the higher up the back of the neck it goes, the less pressure your fingers can exert on the strings. So move the thumb down, towards the floor, and place it lower than the half-way point, but also closer to the head of the guitar.
We're all built slightly differently, so you will have to experiment to findd an optimum position - which will change subtly depending on what chord shape you're using. And also how far up or down the neck you're playing.And where you wear your guitar. The lower it's slung, the more problems you'll have!