Could my brain fog and dizziness that i've had for three years, be a brain tumour?

Unlikely: A brain tumour causes severe problems in short time, as the skull is a very rigid structure and the increase of its inner pressure is a cause for encephalopathy , ie, fogginess, diziness, severe headaches, vision problems, confusion, agitation, weakness, ..and many more. With your history, is important you have your glicemias in check and see a neurologyst .