I always get canker sores inside my mouth everysince I was litte. Very painful. How do I stop this?

Boost immune system. "canker sores" are a loose name for open mouth wounds. The recurring open sores are commonly herpes simplex viral lesions. They are reoccurring when your immune system is low due to stress, poor dietary habits, sleep loss, other chronic illnesses in the body and vitamin deficiencies. Look also to your oral hygiene and certainly improve it while they are present. Don't irritate them. See a dentist.
Possibly laser tx. If it is trully a canker sore you are getting more and more research shows that laser treatment of the sore right when it is about to start, at the early tingling sensation phase can get it to disappear faster, and overtime their reoccurrence is decreased.
Valtrex (valacyclovir) Its now more affordable generic, but need a prescription so talk to your doctor about preventative treatment.
Medication. Unfortunately you can't really stop this but if the sores you get are very bothersome you can ask your dentist or doctor for a prescription to reduce the pain or a topical steroid to help it heal faster.