A: Yes you will die, at least your physical form in it’s present state will one day cease to be. It is part of the process of life. Religion or spiritual philosophy may help lessen the dread of such an eventuality.
As for your jaw, it’s probably temporomandibular (jaw) joint disorder (TMJ). I have a bite guard for sleeping that I no longer have to use. Mine passed with care and treatment, but do address it before it becomes acute or chronic.
I’m not a doctor or dentist, guru or minister, so have it checked out by a medical professional. You may be grinding your teeth in your sleep. You may have unresolved pains or psychological issues that are causing you stress.