You sound like you have depression. Most people with depression do typically feel they are alone as in the only person in the world who doesn't do XYZ even though rationally it isn't true.
When I have my depressive moments I feel totally alone, like I'm trapped all alone in a dark place where everything is dead and there's nothing but darkness and thick fog and dead trees and wasteland and there's no way out. I feel like the only person in the whole world who is there while everyone else is in a lovely, happy, colourful, noisy, beautiful, lively place.
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Female