This is because of the upper third molar .

The soles of the feet of the people who do not wear footwear is thicker than the people who always wear shoes or other footwear. This is called as keratosis and is basically a defence mechanism of the body to highlight chronic injury. So the similar chronic trauma in your mouth will cause similar lesions, as is the case currently because you have been avoiding the treatment for a very long time because the symptoms have not been extreme. As pathologists, we have a practical approach to diseases. You can spend the time in understanding the disease which has already happened and the causes behind it later,  but the practical approach is to get it treated in time before it can turn into something more aggressive. So I will be blunt enough to give you an honest suggestion to visit a dentist for the appropriate treatment rather than looking for detailed answers on an online platform. This is in concordance with your question about this being dangerous.... It is not a cancer at present but any chronic trauma is a potential source of cancer and that is an established fact. So better get it treated before it changes its intensity. Regards.