I get mouth ulcers all the time. It stays for a few days and then disappears. What is the prevention and treatment for it? It gets very painful.

Mouth ulcers. This question is best answered by a personal evaluation by your dentist and physician in order to determine the specific types of ulcers you get and your complete dental and medical history. There is not a simple answer to your question that can be attained by internet research or even posing that question on this website.
It depends on why... Hormones, acidic foods, stress can all effect mouth mouth ulcers. Canker sores can be viral. It is best to let a dentist council you on the etiology and also a treatment regimen.