I can't finish writing a story because I can't imagine it

I've always been in a situation where as I imagine in my mind what I'm writing. Imagining a scene as vividly as I can, writing it down, or imagining characters. But I'm having trouble with what I'm working on now. It's a story about the end of the world. I fail to imagine a fictional world, I just imagine the real world falling apart. I feel like I'm failing as a writer. How does one stop imagining something as realistic in their own life and move it into a fictional setting. Thanks for reading.