Can someone help me dissolve this, and give insight into how to proceed with someone who says this?
What are they saying, exactly? That the set of beliefs in their head that they use to make decisions is not the same set of beliefs that you use to make decisions?
Could I say something like "Yes, that's so, but how do you know that your truth matches what is in the real world? Is there some way to know that your truth isn't only true for you, and not actually true for everybody?"
I'm trying to get a feel for what they mean by "true" in this case, since it's obviously not "matching reality."
In my experience, people appear to have informal, relatively fluid and vague concepts for things that also have formal, precise and rigorous definitions/expressions in systematic thought, shall we say. Truth appears to be one such thing, as I am sure others have noted here. When someone speaks of "my truth" there could be a few things implied or confounded within that declaration
The rational response to such a comment is to issue oneself a firm "nolle prosequi" and exit the conversation politely*. I have no idea what the conversation was about, so I cannot know the truth of things ;)
*if the topic happened to be one with sufficient fogginess in the real world , re-examining one's own beliefs would be a necessary step - heck, I'd re-evaluate anyway, just as a sanity check.