The term nationality itself is only about 300 years old btw...
That is absolutely false. Nationalism itself exists a long time now. Ancient Greek states had extreme nationalism, even between different states. Also, for the longest time, there was also the even more advanced idea of panationalism for a united Greece. Then, Rome came. Do I really have to explain why Rome, that was the first nation persistent on assimilating "Barbarians" effectively into their "superior" culture, was nationalist? I sure hope not. Even in the middle ages we had nationalism. A few examples would be Byzantine nationalism (the restoration of the Byzantine empire after its fall to the Franks was a major thing for almost 60 years), Venetian nationalism (if you read about Venice, you can strongly see their belief that they were seperate and superior, both as a state AND as a people, from the rest of Italy). Or how about Lombardian nationalism that eventually led to Northern Italy being seperated from the Holy Roman Empire (and yes, I know it was a direct consequence of a failed Imperial intervention, however they already were semi-autonomous, purely due to their nationality). Or, how about we move out of Europe? China had strong nationalism elements with many different cultures always seeking as an end goal, even on harsh times, to re-unite into what is called "China" or how about the prevalent belief that China was the state of the god and perfect?
And I won't even get into what is called religious nationalism, which was prevalent within the middle ages (the crusades are a prime example).
Saying nationalism is 300 years old is outright ignorant.
All-in-all nothing good comes from nationalism imo, ever. To be strongly proud of once geographic origin, is for someone who has nothing else to be proud of as a person.
"Geographic origin"? How about culture? How about heritage? How about values? You would never go to Saudi Arabia, even though the educated people there get crazy amounts of money. Why? Because their culture is extremely different from yours. Their values probably seem disgusting to you. Yet it seems normal for them. Nationalism shields you from them and that is a force of good.
However, how about we discuss other direct benefits of nationalism? Nationalism led to the dissolution of the feudal state (of course, nationalism was pushed by the enlightment) and the entry to the modern era, especially after the fal of the HRE. Nationalism has drove the drive of scientific innovation and exploration. The Portuguese explorers are an example of that (and no, don't give me the BS that discovering America and Asia was harmful to the world).
I could go on and on for days on the subject, but yeah. Your views are very very simplistic.
edit: there was even a form of feudal nationalism in the 16th century. For example, Friedrich der Große of Prussia had a king-worship movement around him that could very well be considered nationalist in nature (unity of the lands of Prussia)