First of all, welcome to the site. If you're scared, you came to the right place. I can assure you that everyone who has posted messages to this board either is, or has been, scared. So you're among friends.
Since we're not doctors, there's no way any of us at this site could possibly diagnose your condition. However, I think you would be wise to pursue this with other doctors, especially if the sores in your mouth are more than 2 weeks old. Have you been examined by a dentist? He or she may be able to tell you exactly what these sores are, or refer you to an oral surgeon to see if a biopsy is warranted.
It sounds as though you are linking the sores in your mouth with the enlarged lymph nodes. Maybe you are jumping to some conclusions, based on your reading, that are making you more scared than you need to be. These may well be completely unrelated problems, and each of them may be a relatively benign condition (some type of infection, for example). You mentioned that the swelling has been going "up and down" recently. I have my neck examined by an ear, nose, throat doctor (ENT) every couple of months. He has indicated that cancerous lymph nodes are very firm nodules, and they don't go up and down. Here again, pursue this with your GP if you still have concerns about it.
It sounds as though your GP didn't do a good job of communicating to you why you should NOT be scared. What was her explanation of your swollen lymph nodes? If you're not satisfied with it, I would be honest with her, and ask her for a referral to an ENT (usually listed in the yellow pages as "Otolaryngologist"). Between the dentist/oral surgeon and a good ENT, you should be able to get the answers you're looking for, and the piece of mind you need.
Take heart in the fact that the most likely scenario is that you do not have cancer. But whenever a sore in the mouth does not heal within 2 weeks, prudence dictates that you get it thoroughly checked out. A little inconvenience now can pay big dividends in the future.
Best of luck to you, and let us know how things are going.