TIL that this thing had a name. I used to clench my teeth unconsciously for around two years, until one day I broke a premolar. The reason was mostly anxiety, unhappiness, etc. I was aware of my mental state but completely unaware of the clenching. Surprisingly (or not) focusing on the clenching helped me relax my jaw as well as my mind. Kinda like what is taught in Vipassana ... just trying to be mindful of what the body is doing helped me get out. I still find myself clenching my jaw some times, but I quickly relax when I do.

EDIT: added a missing "body _is doing_"