Unfortunately, the only way of stopping gripping the ball takes a lot of time and effort to rectify. I have had some thumb problems in the past, so I know where you're coming from.
First get your fit checked, as dizzy has suggested, it sounds like you have too much right lateral, or the hole may be a little small causing damage to the left side of your thumb (assuming by left you mean the side that is farthest from your track with your hand in the ball).
In my experience, the best way of correcting a gripping problem (once your fit is sorted) is to do practise drills and use a lot of tape. I find the Dick Ritger foul line and one step drills excellent for this (and the foul line drill can be done at home too). The most common reasons for gripping the ball are poor fit or a muscled armswing although often the poor fit causes the muscled swing.
I would suggest going to practise a minimum of three games a week and only do drills, with time and patience, you'll start to need more and more tape in your thumbhole.
--------------------
Reporting from England