Sorry to hear about your mum. In your position, I would go to a menopause specialist for advice. I know Professor Studd (have a look at his website) would not be worried about prescribing HRT for someone with your family history. In the circumstances, I think it would be worth speaking to someone who knows what they are talking about and I think your particular case might be beyond the average GP.
Did you migraines start once you reached menopause? Do you know what your migraine trigger is? I starting suffering from silent migraines at menopause and mine are caused by low oestrogen and high progesterone.
There is no point in struggling on indefinitely, it's time to get some good advice from a professional.