The problem I think is that you are starting from the assumption that America had a uniting mythos that only recently fell apart, rather than America having had deep, vicious internal divides throughout its entire history that were sometimes temporary subordinated to even stronger shared opposition to external forces.

> It wasn't always this vicious, or this pervasive in its viciousness, in my lifetime.

In our & our parents lifetimes, we had the "even stronger shared opposition to external forces" of Cold War opposition to the Soviet Union to keep these divisions in check.

America's divides have been terribly vicious in the past (https://en.wikipedia.org/wiki/Bleeding_Kansas, https://en.wikipedia.org/wiki/American_Civil_War), and some of the current things that divide us even have their roots in that era.

And even then, the Civil Rights and Anti-War and other internal conflicts of the 1950s-1970s weren't insignificant.