I don't think you have a brain tumor at all; I would see a chiropractor, they specialize in headaches and such. When there are vertebrae out in the neck region, you can have all kinds of sever headaches, dizzy spells, blurry vision, etc. When my neck it "out" I feel horrible, but, after a treatment, I am usually fine again. It is worth a try, and I bet one could help you.
The vast majority of headaches are not brain tumors. Brain tumors are actually very rare, and headaches can be caused so many different things. Headaches are just one of many symptoms of brain tumors (and they are rarely the sole presenting symptom). Also, in 3-4 years you would have most likely developed other symptoms (or be dead by now) if it were a brain tumor. That said, it might be appropriate to have an mri to help figure out what is going on. You should see a neurologist for an examination - if you haven't already.
Best of luck...