After years of playing, what used to be calluses on my finger tips have turned to very thick skin.
Pressing too hard when playing is bad for your thumb, fingers and tendons in your hand and wrist. As well it slows down the ability to change chords quickly and hurts the tone as well.
It might be worth it to have the set up of your guitar looked at by someone good, just to see if that might be causing you to grip so hard.
There's an exercise I learned studing upright bass that I also use on guitar that helps reduce tension. It involves practising chords and single notes without touching your thumb on the back of the neck. Its a bit difficult at first, but with practice you can find a way to do it. It's just meant to be an exercise, not for performing.
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