... However, I have yet to see a difference between a personality disorder and a shitty personality. They seem to be just labels for a common personality we find unpleasant.
That's because there isn't any. 'Personality disorders' are a clinical label to apply to someone who has certain well defined problems with life, it's meant to be more specific than 'an asshole', but no less damning. Everyone who uses them knows that there's a lot of crossover, but it's a lot more useful to use a bit of jargon between doctors, psychiatrists, and psychologists so they know what kind of asshole someone is, rather than just put 'asshole' on their file.