This is the most challenging edge case, for sure. But if I may proffer:
> the more you try to ignore it or brush it off or search the fault within yourself
It should never come to any of these things. "It's never personal" doesn't mean you put up with unacceptable behavior, nor blame yourself. If some sociopath decides to fling emotional abuse my way, he gets called out on it. Not because it's an attack and I will defend myself (both of which are true, at the limbic system level) but because in the end, my personal integrity requires it: I wouldn't treat others this way, I won't be treated this way, I wouldn't be an idle witness to somebody receiving similar abuse, there are healthy ways of resolving conflict, etc. In this way, it's fully de-personalized: it's not about him, or me, but the values which I'm always free to choose and reaffirm.
The fact that they may make it personal, make it about you -- that's a further reflection of how impersonal it is for them. The psych term is projection, but you don't need to concern yourself with their diagnosis.
There's a reason I paired the "it's never personal" quote with the "you train others how to treat you" quote. The latter is a reminder of your own agency.
Dealing with a sociopath or narcissist is, in some ways, easier. Their behavior is so uncooperative, they immediately forfeit the privilege of your empathy. They clearly have had a terrible emotional upbringing to even arrive at a point where they would so freely treat another person like this. And that's the point: it's not personal, they're just... a fairly broken human being.
People this broken, they can't hide broadcasting their brokenness from a mile away. It makes it easier for you to know who to keep at arm's length with minimal trust.