I have the same problem, but to a lesser degree. I have never bitten through a whistle. But in rather stressful situations I find myself biting down pretty good. Frankly, my jaw starts to hurt too. Like Tim said, you need to find a way to relax. I use the following:
Imagine, just for a moment, that you're in your happy place. There are beautiful, snow-capped mountains in the distance, and you're standing in a lush, green field. You are next to a crystal clear stream. The only sounds you hear are the gentle wafting of the breeze, the burbling of the stream, and a slight thrashing under the water.
You look down. The stream is so clear you can see every feature of the stream bed. There are moss-covered rocks and braches. Lurking near the bank is a large fish, eying you suspiciously. And right next to you, there is a face. You recognize the face. It's that coach who's been giving you grief. He's trying to call out to you; there is panic in his eyes. But you can't hear him. There is a pair of hands, grasping his throat, holding him under the water. You recognize those hands. They're yours.
Ah yes, your happy place.
If that won't help you relax, you may need meds!
