Ive developed what I call a nervous twitch. When I'm worried or upset about something I do this thing where I open my my mouth and then close it quickly, hitting my teeth together quite hard. Unfortunately it's happening a lot at the moment due to certain situations and although most of the time I don't even realise I'm doing it other people have started to notice. Then if someone brings it to my attention my mind is stuck on it until I make myself do it.
How can I stop myself doing this? Especially when I don't realise I am in the first place?