Clinching teeth...is there a remedy?

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I clinch my teeth..not by choice. I do this all through the day and even while I sleep. It goes away after about a week or so, but then a few months later, it'll come back. I don't know why it happens or how to stop it. When I'm going through this, I have trouble eating because it hurts to chew my food. I've tryed those plastic mouth pieces, but they're very uncomfortable. Any suggestions on how to break this habit?