Common causes of cracked mouth corners include dry or cold weather, sunburn, wind exposure, licking your lips, and breathing through your mouth. In some cases, conditions such as the common cold, anemia, dehydration, and nutritional deficiencies may also result in cracked mouth corners.
Apply a non-irritating lip balm (or lip moisturizer) several times a day and before bed. If your lips are very dry and cracked, try a thick ointment, such as white petroleum jelly or pure vaseline. Ointment seals in water longer than waxes or oils. Slather on a non-irritating lip balm with SPF 30 or higher before going outdoors.

Assuming you’ve had it checked, I can only suggest lots of moisturiser.

I used to get this growing up and I think it could be from the saliva. My mother used to put boiled rice on it and it goes away . I tend to use lanolin cream these days and it works better .

I smother Vaseline or paw paw ointment.

Put paw paw cream on at night time.
Because you’re not eating or talking during your sleep it lasts a lot longer than daytime applications.

sounds like it is just dried out and when you move your mouth, you tear it. try to moisturise this.