the subject of death is so terrifying because
EXISTENCE is all we have EVER known. it is the foundation that causes everything in our lives, without it we wouldn't even be a thought. i think that's why death is so relevant in many religions because the thought of simply NOT existing feels like something that is bigger than us, much like a god or deity. we try to grasp it and comprehend it but we can't, so we create religions to form some sort of relief for that impending doom. taking it all in and really thinking about death when you don't have that belief to clutch on to IS scary.
as someone who's non religious and likes challenging concepts, i've come to the conclusion that spending our limited time in existence worrying about the day we stop existing is nothing more than a waste of time. why be scared of eternal darkness when right now your blood is pumping and your brain is thinking and you've got work in 30 minutes? sitting there trying to conceptualize death will only get your further from accepting it. get up and brush your teeth and go for a walk and listen to music and call a friend. YOU ARE NOW!!!!! and when the time comes that you are not, you won’t be there to worry about it.
even just for general things in the future giving you anxiety, you have to tell yourself that you are now. YOU ARE RIGHT NOW AND RIGHT HERE AND THAT IS THE ONLY THING THAT MATTERS!!!!!!!!!