First, calm down! Any sore that comes and goes is likely not cancer. If a sore has been in your mouth for at least 2 weeks and not gone way then its time to get checked by a professional. I would suggest seeing an ENT (ear, nose and throat doc). I would doubt that a cancerous tumor would be in your mouth for years without you having any other serious consequences.

Smoking.... you probably already know what Im going to say. QUIT!!!! Let this be your wake up call. There are hundreds of chemicals which are irritants added to tobacco products. Any one of them alone could be whats bothering your mouth. Many of these irritants are known cancer causing agents. I am a former smoker myself. I quit cold turkey when I was on my way to get a feeding tube. Stupidity is why I am now disfigured and live a very different life than before I got sick. I almost died all because I smoked. Dont let this happen to you. Call 1 800 quit now or check out the CDC's Tips From Former Smokers website. You can see what I look like and why I am telling you to quit. I wouldnt wish oral cancer on my worst enemy!