That's honestly very understandable, and I do man it lol: the fear of the underworld is nearly as old as civilization itself. A Mesopotamian scribe, referring to the gloomy view of the afterlife they had as compared to the Egyptians who had a happy view of it, writes: "Tears, lament, anguish and depression are within me. Suffering overwhelms me. Evil fate holds me and carries off my life. Malignant sickness bathes me."

It is hard to shake off a notion that has been embedded in you since you were young and which, most likely, your human brain itself compels you to believe in, i.e. it prefers to think of hellfire than absolute nothingness, continuity with everything that has ever come and been utterly forgotten, even if you rationally understand it to not be real. I don't know you at all obviously lol but I would bet that there is a masochistic element to it too most likely: you probably feel that you should be punished, rather than just thinking you will be post-mortem, and that is why your brain is making you think of such things. I've seen this very often with people who believe in hellfire damnation awaiting them, and more secularly, of people who plain and simply want to be tortured.