Our mythology has always been life, liberty, pursuit of happiness. Freedom of speech and religion. Democracy. Rule of law. They aren't as radical and in vogue today but this was and still is our "mythology", IMO.

I think that's an idealized interpretation of a young country that spent it's first 100 years believing humanity is attached to skin color, fought a brutal civil war over that before spending its next 100 years preventing most of them from entering commerce, and its last ~50 years incarcerating as many of them as possible. And things like shadow wars in Cambodia do not look good on a country proclaiming "rule of law". So perhaps that's how you perceive it's shared methodology, but certainly not "how it has always been".

Mythology does not equal reality. It's a target to aim for. No countries "mythology" matches their reality.

My theory is not that mythology kept you together, but that there was enough cultural and physical space to peacefully ignore each other if anyone so wished, simply by moving a bit further away. Communications were slow enough that they could be digested or ignored more or less at will.

The problem is that, between natural population growth and the rise of the internet, both the cultural plane and the physical one have largely run out of space. Figurative "midwest zealots" just cannot live their lives ignoring "coastern heathens" even if they'd like to, and viceversa. And so the fights have broken out and will likely continue for a long time.