I've never been seriously suicidal, but I did have a period of bad depression/OCD where I experienced what I later learned is called "derealization", where I felt cut off from everything in life and as though I was viewing the world through frosted glass. Once I received help and left that state, I was struck by how much it felt that everything I cared about was already gone. I can only imagine there's a similar feeling when people die by suicide.