Hi, Allen,
Yes, if the cognitive side effects were going to stop, they probably would have stopped by now. It's time to make an appointment and have a talk with your doctor.
You didn't say what, if any, other preventives you've tried, but keep your chin up. There are now over 100 medications that can be used for prevention, so you still have options. Take a look at Migraine preventive medications - too many options to give up!
Good luck,
Teri