How to stop clenching jaw (not grinding)?

I don't grind my teeth, but I clench my jaw a lot, just out of habit, like how I bite my nails and crack my knuckles and twirl my hair without thinking about it. But once I realize I'm doing it, I release the pressure I'm putting on my teeth and my jaw hurts. I'm trying to stop, but I can't. It's not like I can wear a mouth guard or something, because I do this all the time, like at school and stuff too. Chewing gum helps but I don't like gum. Biting my nails distracts I think, but my nails are worn out from that. I don't know why I do this, I think its just a habit, but it really hurts my jaw. How can I stop?