Sure, World War 2 ended much more recently than the Civil War in the USA. But I don't think the recency matters all that much. More importantly, I think Europe did a much better job of "healing" after the war was over than the US. Even after the Civil War ended, the bitterness of the slave states ensued, and continues to exist even today. You could probably argue that this continues to happen to some extent with WW2 in Germany/other parts of Europe today, but nowhere near at the same scale as the analogue in the US.

To me, the USA still looks like it's a country that's still at war with itself. It just happened to go from a cold war to a hot one in the 1800s, but there has always been a divide between slave state and non-slave states and it feels like it has never ended (and probably never will).

So yeah, I would argue that your last point is spot on. The USA looks like an inherently flawed country to me - it's too big and too non-unified. It just looks like it would make so much more sense if it were split up into a few smaller countries that would be much more unified and cohesive.