Try this little experiment;
Hold your bow arm out as if you are holding an imaginary bow. Chances are that your knuckles are in the verticle position( up and down)
Now turn you bow arm with the imaginary bow so that your knuckles are horizontal ( going across).
Notice what happens to you forearm and wrist.
You should be holding you bow with your knuckles almost horizontal. Only the heel of you thumb should be touching the grip NOT YOUR PALM!
This is the only part of your hand that should touch the grip.