I agree with OutOfMyMind75. Find something to occupy your attention, like a long series. I remember one time being extremely upset about something and I ended up marathoning Inuyasha for 3 days. That marathon felt as long as a lifetime and put a real feeling of distance between me and my agitated emotions. From there, just try to hold onto it and gently steer your thoughts away from those toxic circles.
About obsessing about death... I get it. I have periods like that too. It will just suddenly seem so real and terrifying, the fact that it's coming. All I can say is--in my experience, those periods don't generally last. You can only spend so much time feeling terrified of something that's always with you. It will fade as you become more engaged in other things.