I worry about dying young.

I'm 24 and about to be 25 I'm constantly feeling like having a panic attack even fast heart beats. I know death can come at any moment and it's just that fact alone that someone can just kill you. It's the fact I don't go anywhere that kept me alive. I hope to be alive from an older age but I haven't done anything useful which I'm sad about death. I'm sad I smoke as well I'm sad that this means I've mostly killed myself. You know what actually does happen? If death is just like sleep that might be peaceful but there's not really another chance at living. If it's something then it's like I might see people I don't like I miss people in some way but not being able to see them again because they committed suicide or some other stuff. I don't get it anymore actually and just kind of wait which I hate.