For me, it's the fact (?) that the universe will end in a heat death at some point. Eventually, everything, EVERYTHING, will stop. What happens after that? Infinite nothing? It will probably never concern me, but that doesn't stop my mind.
Then there's death. I don't want to go. I try to imagine what it was like before I was born, but the blankness of it... I don't want that again. I think, if I have a long, non-worthless life by my own standards, that I'll be okay with it when it comes, but there's no guarantee of that. Oh well. I can still function, so that's good.
I hate my fears.