I'm so sorry. From your describing her situation, it doesn't sound good. A brain tumor is not going to get better and the frequent seizures can do more damage to the brain.
If it was my dog, at this point, I'd probably make an appointment with the vet to help her along to the other side. It's a heartbreaking thing to do, but it's also a selfless thing to do.
For me, it's when they no longer have a good quality of life. My male that died of CHF, has some backstory, but the inexperienced vet that was left on duty during New Years Eve, did not take the additional xrays or even compare the xrays she took that day to previous. He died in my arms, around 3am on New Years Day on the way to the ER vet to have him euthanized. Absolutely miserable experience.