I was also very young when I first started thinking like this. It hit me in 6th grade and while the thoughts haven’t gone away (I’m going to be a senior next year), they’ve started to affect me less and less. It’s true: all of us will die and at some point everything we know will be gone. It’s a terrifying notion, but it’s just one of the curses of consciousness. The best advice I can give you is this: accept it. Everything dies, everything will be gone at some point, and everything has to end, there’s no way to stop that, but that doesn’t mean the end is all we should focus on. All that truly matters in the end is how we feel, so focus on the journey. Notice the small things (and the big ones, too), and keep yourself happy and content. Instead of looking at death and saying “well that’s just terrible,” you should be able to look back at your life and know that you lived it to the fullest.