I'm sorry you've been dealing with this for so long. Have you seen a doctor? If not, you really should.
This could be caused by a lot of different things - it could be a migraine, low blood sugar, low or high blood pressure, or many other things. It's impossible to know without some testing and maybe some imaging.
If it's bad enough for you to have to pull over - meaning it is interrupting your daily activities - then it is time to see a doctor.
Let us know what happens!