I want to die but I’m scared of what comes after

Ever since my nan died when I was 8 I’ve had anxiety attacks about death. Not about the actual dying more more the idea that I am going to grow old and the world is going to go on and I just won’t exist. Just typing that make my chest tighten and I feel like I’m having a heart attack.
However, recently I just feel like I’m never happy anymore and the world has nothing left to offer me. I have no motivation and all I want to do is sleep or do things that make me feel like I’m part of some fantasy world like video games or books. I already self harm and I’ve tried stopping but the relief I get from it just helps me get through the day. If I wasn’t so scared about not existing I probably would have ended it ages ago but I just can’t. I don’t know what to do, I tried to see a doctor but they said it was hormone imbalance, probably because I’m 17. I tried telling my mum but she said it was just a teenage thing and I told my friends about the self harm but they either act like I didn’t tell them or like its not serious enough for them to care that much. The truth is I don’t really want everyone knowing how I feel or bugging me about this but I just can’t live like this anymore. I feel like such a mess. If someone could help me that would be great.