How can I stop lip and cheek biting?

Sounds Like. A habit that you have formed and like nail biting most likely is an unconscious habit. You will need to be patient to change this behavior. This will take 6 wks and you will have to be diligent and pay attention to the times and conditions that cause you to bite lips/cheeks. Changing those conditions, being aware will work. If not see your md/behavioral psychologist.
Habit hard to break. This type of habit is not easily changed, but actually wearing a thin mouthpiece may help, where while wearing it (day and/or night, ) it can possibly help retain your habit to stop. Trying to just being aware of it often doesn't work.