Oddly enough, it may be that your thumbhole is actually too loose. This forces you to grip the ball more than you should and it's very hard to time when to let go. Another possibility is that the thumb pitch needs to be changed. You may have too much reverse, also causing you to grip too hard. There's a myriad of possibilities.
Personally, I use a piece of magic carpet in the back of my thumbhole, and, after working with Ron Clifton, the inventor of it, about 4 pieces of white tape in the front. I would never have guessed that my thumb was drilled that oversized, but it was. I have to add tape as the night goes on, too, because my thumb shrinks as I play, and if I don't I start gripping and hanging up in the ball.
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