Believing you are dead when you are alive is actually an illness called Cotard delusion.

I actually went through a state of this and was sent to a lot of psychiatric facilities because of this. I felt like I was not real. My reality was distorted and I thought I was dead and in the afterlife.

Anyways, sometimes I still lose grip of reality. It's fleeting, but super weird. Sometimes I wonder if my suicide attempt actually didn't fail or something...