To be fair, reports of our "nationalistic" tendencies are somewhat overblown. To begin with, our war for independence was fought not because we felt we were different or superior, but rather because we weren't granted the rights and privileges that proper British citizens received. There were few who desired independence before the long string of grievances (proclamation of 1763, stamp act, Townsend acts, intolerable acts etc); rather, they wanted to be properly integrated.

After becoming an independent nation the U.S. has certainly had an independent streak, but I don't feel they were any more "nationalistic" than most western nations. Unlike many, from the start they wanted to reduce the popularity of mercantilism and open the world to trade, and the wealthy trade relations established between the U.S. and UK almost immediately following independence, it being more lucrative to the British than when they owned the territory themselves, was an early model for less restrictive trade going forward.

Otherwise, the U.S. also overcame its own strong, powerful prejudices towards other cultures and religions in allowing unprecedentedly large legal waves of immigration. Really, I marvel at the idea that in 2015 the very idea of accepting a few thousand refugees is meeting such resistance in many modern, enlightened European countries, considering the U.S. in a bygone era (1830-1920) accepted over five million Irish Catholics (given that's nearly my entire ancestry I'm glad for it, and don't underestimate the Protestant hate for Catholics in this era). 

They were obviously treated terribly for the first few generations, but the idea that just one group of migrants, possessing a different culture, religion, no money, and even often language (Gaelic was not uncommon) was allowed in to the number of over five million simultaneous to many other groups, I think, is quite remarkable. To this day we still legally accept slightly over a million migrants annually (too few in my opinion), far make than any other nation, and still make the same fear-based complaints about said immigrants that have been spouted since Ben Franklin worried aloud that we might soon be speaking German.

I think, then, that this is a misappropriation of the term "nationalist". We've not had nationalistic tendencies since we were fighting for our independence, and even then it was in pursuit of political rights that had been denied. The USA has long been a messy country that is, essentially, inventing itself on the fly. Our politics, demographics, and economy is in a constant state of flux, though politically we've largely seen more stability than most nations since the end of the Civil War. There is no real unified and readily accepted concept of what "America" is here quite like there often is for older nations whose population has hardly changed over the centuries.

Otherwise, your example of us not adopting the metric system is the result of convenience, not obstinacy. The U.S., with its geographical size, over 310 million people, and 50 states, is like an EU unto itself. Unlike the EU where cooperation and conformity were enormously beneficial for the shared trade, roads, and highways of a great many countries, no such need exists here. We have two neighbors and the borders are not open.

It simply makes little sense to make the switch. In all matters science the metric system is used and it is taught in our schools, but the public is familiar with the old imperial system and uses it in their everyday lives. There's just little reason to convert for the average day activities. Consider, for instance, how costly and time consuming it would be to replace all the signs of the world's longest (by far) network of roads with metric measurements, and then retrofit all the dashboards with km/hr to avoid confusion in a nation with more cars per person than any other spare the micro states of Monaco, San Marino, and Lichtenstein. Who, really, would even benefit from that? It would simply be a waste.

Really, we have less a sense of ourselves than just about any country, and we've not gone nearly so far down the road of true nationalism as many countries did throughout the 20th century. People just look at us and see that our measurements are different, our sports are different, that we often place greater significance in what the constitution says than an international body (to the point that we'll propose something like the League of Nations and not even join it lol), and they think these are deliberate attempts at doing the opposite of the rest of the world. This, of course, is largely untrue; the idea that we could so come together on any idea as to be capable of pulling off a grand plan like that is silly. Rather, the US simply grew from the ground up as a nation with few anchors in the past, no real rivals or puppet-masters in the region (especially as they approached the 19th century), and few connections to the old world. That it wound up looking different from other countries should not come as a surprise.