Could it be a misunderstanding? Maybe what your doctor means is that you might have many lesions that for other people could mean more disability and being worse and that in fact maybe you are lucky that these lesions are not affecting you that much as they are in parts of the brain or spine that are not used that much.
Anyway in my opinion what does matter is how do you feel, even the same disability for one person could be terrible and for another to not to be that bad (for example a drop feet for somebody that loves running vs a drop feet for somebody that loves watching Netflix).
As for your thoughts of course these are normal, it’s terrible to know that we MIGHT die sooner that the others or that we will be more disabled but the truth is that we just have more information about our future, while the person that tomorrow will have a car accident and will be totally disabled doesn’t know it today and of course that person would be also thinking about assisted death today if he would know about it. So don’t feel bad about these thoughts but also try to have support from the people around you or Psicologist or group therapy, whatever helps you.