There are a number of things that can cause grand mal seizures (low blood sugar, drug interactions / withdrawal, head injuries, epilepsy, etc), but anxiety / panic is not one of them. If the medication you are currently on is not working, you must let your doctor know asap. These seizures rarely cause death... so you don't have to worry about them killing you in and of themselves. However, they can certainly cause injuries from falls and such. I agree with ILADVOCATE, you should have a full neuro work-up with and EEG and imaging (MRI w/ contrast) if you haven't already.
All the best...
Keppra is an anti seizure medication so if you are having grand mal seizures then they might need to change it to another medication or add another. Anxiety can worsen any disability but would not by itself cause seizures. You would definitely want to speak to a neurologist to get a full check up including an EEG, MRI and perhaps a sleep study and in the meantime post the question in the neurology forum as well.