I remember when I was young (12-20 y/o), I used to wake up and laying in bed thinking "why am I a human being? I will die someday, and where do I go after that? Will I still feel/see this world?" and I got so scared that my whole body became numb. The feeling usually lasts for about 5-10 mins but I can't get rid of the thoughts. I still think about things like this now (28 y/o), but rarely have panic attacks anymore.

For my own case, I used to always think about it extremely scientifically that "when we die, our brain is dead, so there will be nothing. We can no longer feel or see or think anything. It is just nothing and we don't exist anymore." I think that's why I always feel so scared. But what changed, even just a little bit, is that my dog died 2 years ago. Since then I started to feel like when I die I might see her again, which gives me some hope and comforts me about the "emptiness" feeling of death.