Hi, I hope this is an ok subreddit to post this in. Online advice for this issue, including on Reddit, has been generally hard to find or unhelpful. Generally a lot of commiseration and "have you tried chewing gum?".
So here is my advice on how to stop chronic cheek biting, (morsicatio buccarum, a form of body focused repetitive behaviours (BFRB)) from someone who did it all day, every day, unconsciously, for over a decade and stopped in two days. I put it in sections so you can skip to the parts you need. And of course I know my experience isn't universal so there are probably people who have a cause of cheek biting that is different from mine so this method may not help, but I thought it was worth sharing.
Background: I have had this habit for ages, chew the inside of my cheeks and my lips leaving large patches of rough skin that I then need to chew more to make it smooth again, vicious cycle. I have had dentists comment on it because I have scarring in my mouth and linea albea. When I asked them for advice they said try just being mindful and stop, and also suggested a night guard for night grinding. Not super helpful considering they are the experts, but I did get the night guard. Because I did it unconsciously it felt impossible to stop during the day.
The method: The trick is habit replacement and decoupling, two proven methods for BFRB. With habit replacement you interrupt the movement with another movement that uses the body part you use for BFRB to stop yourself from doing it. Decoupling is starting the movement, then redirecting the movement at the last minute to another part of the body.
Basically I was googling how to stop doing this (again) when I found a scientific paper about a woman who had the habit for 40 years and stopped in two days by using these methods she found in a guide online. The guide is also helpful because it gives you a full PDF on these methods, you just have to give your email to get it. I recommend the guide for a more detailed description of habit replacement and decoupling than what I'm giving here.
Inspired, I tried her methods. This part of the paper is paywalled so I copied the important part here.
"As mentioned before, in the decoupling technique the individual is asked to perform a movement that corresponds to the initial approach sequence of the undesired behavior. Shortly before the (undesired) behavioral goal, the affected person is asked to redirect the movement and target another part of the body with an accelerated movement. In AO's case, the movement that initially corresponded to the unwanted behavior was the harmful use of her teeth on tissues in her mouth (lips, cheeks). Instead of the unwanted behavioral goal (biting off the mucous membrane of the mouth with the teeth), AO redirected the activity of her teeth in such a way that they rubbed quite vigorously over the upper and lower lips as well as over the tip of the tongue. This created relief in AO. Both the teeth and the tongue were in use, but they were prevented by the alternative movement from carrying out the undesirable behavior (feeling the mucosa and then biting it). The behavioral irritation was achieved by consciously perceiving the lips and the tip of the tongue due to the blood flow caused by the rubbing."
Habit replacement for cheek biting is putting your tongue on the roof of your mouth for 1-3 minutes whenever you get the urge to bite.
Decoupling for cheek biting is rubbing your teeth vigorously over your tongue or lips without biting.
Within less than 12 hours of using these methods to stop biting the inside of my mouth started shedding like a snake. I had to be very mindful not to bite off these pieces, just to let them fall off naturally so I didn't damage the inside of my mouth again. The worst parts around my lips on the inside of my mouth stopped peeling within two days. I literally did not know that the inside of your lips could feel this smooth. And from there the urge got easier to control. I still found myself occasionally biting down when stressed, but I just reminded myself to use these methods. Its been a month and my mouth has almost completely smoothed out except for the linea albea and I dont get the urge the way I used to anymore.
If it doesn't go as fast for you I would encourage you to stick with it because any reduction in this habit over time is good! Anyways, I hope this helps someone because I know its a rough habit to have.
Other things that may be helpful:
Meditation: I think its important to note that when I started this I had been meditating everyday for 5-15 minutes for about a month. I am no means good at meditating, I get distracted a lot but I think it increased my baseline level of awareness of when I was actually doing this enough to allow me to stop. I do habit-stacking with this where I do it every night before bed so I don't forget, and do 5 if I'm tired and really not wanting to do it, but aim for more time if I have the energy.
Night guard (if you grind your teeth at night): If you do grind your teeth at night or chew on your cheeks at night it might be worth getting a night guard so the irritated tissue from night biting don't make you want to bite your cheeks to smooth them out during the day. There are cheap ones at the pharmacy if you need it or if you have insurance you can ask your dentist if its something you should try.
