If you could quit one bad habit instantly without difficulty, which would it be?

Oh, I would choose chewing on the inside of my cheeks. Back when I had braces, the metal would irritate the skin on the inside of my mouth and I developed the bad habit of chewing on it. Sometimes I am not even aware I’m doing it and since it’s bad for my skin and put the most moronic expressions on my face; I would love if it would go away.