I can't stop nailbiting

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I have this habit of biting my finger nails which dates all the way back to my childhood. I think this is a really bad habit and it is fast becoming inapropriate since I am already in my mid-20's. Although significantly lessened, I can't seem to stop it especially when I take my exams, or am nervous, or when I catch myself thinking about something. Whew. This is kind of just me. Very me. Anyone around who nailbites like me? Mind sharing your ways of struggling against it?