I had this same exact problem a few years back and on a recommendation of a friend bought a box of toothpicks for .59. I would place a toothpick between my thumb and pointer finger of the hand holding the bow, and the first time I tried to shoot I almost drove the toothpick through my thumb from trying to grip the bow.
An excellent idea, it wouldn' t work for me with my fat stubby hands, I shoot totally open handed, heck I couldn' t even get my index finger and thumb together around my grip
]