My sister was diagnosed with depression and anxiety when we were both in high school. It would knock her out for days - she wouldn't get out of bed, shower, go to school or sports practice. Looking back, I realize that our mom would have similar spells but nobody really talked about it. What if this happoens to me? I am afraid that I won't have anyone to support me, and I'll be deserting my sister.
Don't worry too much. Just think positive of life and as long as you have someone with you then it won't happen.
I also have the same worries as you do. My brother was also diagnosed with dementia same with my mom a few years ago. I heard it can be passed genetically.