I am no expert on this topic but I hope that I can help a little.
I assume that you have been all through the full diagnostic process and there's no doubt about the seizure types and likely cause so that everything has already been properly investigated (at least so far as its possible to do with epilepsy).
I assume that the doctor is correct in the interpretation of your daughter's brain scans? I know this sound stupid but with CP, her scans will never have been normal and will always have shown damage. Has this doctor seen the full series of scans to compare them? Is the doctor correct in saying that they show further brain damage? is that damage permanent or merely showing temporary changes which can occur in some situations (eg chronic pain can show shrinking of brain matter which is reversed if the pain is treated). Has the doctor ruled out any other cause of this damage? I don't expect you to know the answers, I'm just suggesting the kind of questions that I would expect a doctor in such circumstances to be able to answer.
You say you feel overwhelmed. Its bound to be overwhelming caring for a child with disabilities and everyone in a situation like yours needs support and help. CP and epilepsy are both relatively common conditions and its also very common for children with CP to have epilepsy. I am wondering therefore, if there are any local support groups you could go to so that you can talk to other parents in the same kind of situation. You might be able to get good medical tips and suggestions from other parents or specialist CP charities and at the very least you would get emotional support.
The idea that seizures actually cause further brain damage is highly controversial and it is by no means a settled medical 'fact' that seizures cause any further permanent damage to the brain. I think you should be pushing your child's doctor really hard with some very probing questions on this point because its hard to understand why he thinks that this is happening to your child when there is no agreed medical evidence that it even happens. You may want to consider consulting someone who has more specialist expertise. One hypothesis is that the epilepsy medication causes brain damage, another is that such further brain damage has a different cause and it is this that may cause the increased seizure severity/rate. Maybe it would be worth you doing a bit of researching yourself so that you can 'interrogate' the doctor a bit harder.
You may have to push for trials of any further drugs that have not yet been tried or combinations of those drugs. As I'm sure you know, often seizures can be better controlled by taking multiple meds. Of course, this may cause more side effects but you won't know until you essentially 'experiment' with the options. It is a good idea to keep a seizure journal and look for any patterns or triggers but also so that as you experiment with any meds or combinations of meds, you can keep a record of exactly how your child responds. It means you don't need to try and rely on gut feel or your memory which is likely to be really difficult when you are stressed and worrying about all of the other day to day issues.
Ultimately you will need to decide what level of side effects are acceptable for the amount of seizure control conferred by any given medication. It may be, for example, that if the current situation is the best outcome possible, it's better to try and find practical ways to manage the balance and falling problems than to abandon those meds. This is something I think only you can decide after you have tried all of the drugs and combinations of drugs out there. The issue of side effects versus therapeutic benefits is one that all of us with chronic, medicated conditions have to deal with and there are no right or wrong answers.
A ketogenic diet may well be worth trying as it is well known to produce remarkably good results in many people with poorly controlled seizures especially children. There is lots of info about this out there but I'm sure your child's specialists will be able to advise on this.
I have no idea if its suitable for your child but vagus nerve stimulation surgery is another option. The device is implanted and sends bursts of electrical activity to the vagus nerve. Although its relatively new, there is quite a lot of info out there on the surgery and equipment.
I am sure you already know this but it can be very difficult to obtain good seizure management in children with CP. It may ultimately not be possible to obtain it. You might finally just be faced with the fact that all you can do to find ways of managing life with the increased seizures and the difficult medication side effects. However, I think there are a few things you can at least look at and certainly considering whether your child's medical team are expert enough in her complex case is key.
Hope that was some use.