Given your past history, I would say your anxiety has a real basis. Wow you have been through some real crud and so yes, why would you not fear something new?
I have been through no where near what you have (a couple of pit tumors, breast cancer, meniere's, myasthenia gravis etc) and even so, when I get a new thing, anxiety is raised. You have coped with a lot of things in the past, so you know time, I think, will lessen the fear a bit. In the mean time, reaching out to others can help for a while maybe your neurologist knows a specific psych doc to help that is aware of the physical as well as the mental (so they don't just toss pills at you and don't really understand).
I am dealing with a post-op infection so not posting as much. Can't wait until this is over.