How can I convince my doctor to give me better pain killers?

You are not going to be able to convince your doctor of anything, sorry to say. Many GP's are uncomfortable prescribing narcotic pain medications for treatment of chronic pain. You should ask him for for a referral to a pain management doctor. They will be able to help you find a combination of medications that work to alleviate your pain. They will, however, want more test run to find out why you are having so much knee pain. If you have not already had them, they will most likely want you to see an orthopedic surgeon and get an MRI of the knee. If the pain is being caused by ligaments, cartilage, or arthritis, it will not show up on an x-ray. If it is a bone problem, then you may not even need the MRI, but I would not count on that. Have you had any testing done or seen any specialists for your knee? Any physical therapy? If not, it sounds like you need a new primary care doctor. It is not normal to have significant knee pain and the doctor does not refer you for testing or to a specialist for a more in depth examination.