Its so important to talk this through.
Do you have the opportunity to have go to any of your DMs follow up appts? You could ask the consultant yourself. Would that convince you?
What would you need to happen in for you to relax about this? What would you need to hear and from whom?
An aunt of mine collapsed suddenly, twice, second time she was literally taken straight to mri and theatre because of a large and deep brain tumour.
They removed it, after the op she couldn't move and they didn't know if she would ever fully recover.
Its taken years, but she has! She is better. It was a specific type of tumour that was very slow growing, non-malignant, and she could have had it from childhood they said.
Its gone.
Shes fine, and just has routine check ups.
Certain types of illness do have genetic links, but you can only know if this applies to you from asking directly. Then, if there's no genetic link, you can rest easy for anyone else, or indeed yourself.
In the meantime do all you actively can to relax, even things like directing the hot shower on your shoulders and top of your back can help a lot to easing tension headaches. Getting some regular daily exercise, and learning to relax your body.
Hopefully now you have at least a better counsellor you can also find good ways to get through the extreme anxiety around this. To move to a calmer place of understanding and acceptance.