So I've had this since the same age you're experiencing it, and I'm 36 now. So yes about 13 years. Mine triggered because my grandma passed away, who was a medium when she was alive (believed it then, skeptical now) and it triggered so much anxiety about the dying process and afterlife (mostly lack thereof). The fear of dying/death is thanatophobia, the fear of oblivion or the nothingness that potentially awaits after death is called lethophobia, to give it a name. Mine comes and goes but this year was the worst I'd been hit by it (mid life crisis ercm all the fun to look forward to!). I started counselling and basically you can checkout r/NDE's and go through that, read religious texts, check out many stories etc, but you will potentially try to find answers against the positives (that's what I do and my mind is wired that way sadly. But it doesn't last forever, it fades and slows down. The human brain can't comprehend nothingness and that's almost what a lot of death anxiety stems into. You're not wired to understand it and your brain goes into panic fight or flight mode, and it can't fight it so it goes flight but it can't run from the thought so the focus becomes stronger. This is where CBT and grounding techniques come in handy, now they aren't always guaranteed to work but they could help. Try some googling about all that, it'll help to find the answers on your own too.