I don' t have that problem, so I don't have a great answer, but rather a few questions/comments -courtesy of talks with other divers who also chew through their mouthpieces:
Do you grind your teeth while asleep? (there might be a correlation)
Does your mouthpiece feel comfortable- almost to the point of you not being aware of it. (if not, try a "custom fit" one-. Trim the edges as needed to get a better fit, try one of the (I don't recall the brand) mouthpieces that let you make a bite area fitted to your teeth[ you put the mouthpiece into hot water, bite it then when it cools it has a "dental impression" fit.
Can you recall when you are chewing the mouthpiece-heavy current, high exertion situations, nervous-inducing situations? Or "boring" parts of the dive (if there are any).
2 things that might help???: (they are from people that said they had that problem.)
Can you consciously remind yourself to NOT CHEW the mouthpiece? (one guy said that this worked for him.)
Another guy said that he lined the bite area of his mouthpiece with a small strip of chewing gum in a flavor that he didn't like , which gave him immediate feedback that he was chewing on the mouthpiece.
I have my doubts, but both people claimed that their method worked for them.
Good luck with your situation.