Hi msf!
It is a posibility for your doctor to increase your dosage. It is also a possibility that he/she can add something different into your preventive plan.
There are over 100 preventive options out there, so don't give up!
I recently went through the same thing on Topamax so I know where you are coming from! In my case I had a new diagnosis, but that may not be the same for you. Talk to your doctor, bring up your concerns, he/she should be able to work with you.
Good luck!