What Causes White Patch Inside The Mouth?

dear Dr. I am 59 yrs male since last 3 months the cheek area adjoining wisdom tooth on left side and molar teeth developed white patch which caused pain during opening of mouth. now it has thickened a little.I have now less problem

while opening my mouth.I used to eat pan masala with tobacco now I have stopped .My querry is will it be okay after stopping tobacco& maintain good oral

hygene.