You might think that no country has any right to exist but that belief wont protect you from someone who thinks their country should control the entire world.
Nationalism is a basic and maybe outdated mechanism that protects against foreign invasions. You need to understand its purpose and function if you want to abolish it. At some point, i think everyone would agree that some populations share certain moral values and priorities. And those values deserve to be defended.
You can call those values "Germany" or "EU" or "Europe" or "lemmy" or "lgbtq people of earth and nearby planets", but in the end you will have an entity that encapsulates values that are worth being defended. I dont give a fuck about my country and i wish daily that somehow it gets nuked out of existence. But fuck anyone who wants to take it over by force.
Why is all this important? We are entering an age where information warfare is crucial. If an entity can shield its population from enemy informational warfare(great firewall of China) while being free to use informational warfare against its enemies(tik tok or any social media), then you(in the West) will just become a useful idiot.
The enemy is trying to persuade that a thing is bad(countries/nationalism), while at the same time is preaching how great that same thing is on his own population. When the shit hits the fan, which side do you think will win? The one who thinks that nations are bullshit and no war is justified, or the country which thinks that its nation is the greatest and war is a moral necessity in order to "liberate" the rest of the world?
This is nothing new. Japanese imperialism was disguised as anti-western/anti-imperialistic. Japan was the sign that asian people can be equal/superior to europeans and they just wanted to spread their values and liberate other asian nations from the european/american shackles. You dont need to be a historian to realize how utterly bullshit this lie was. The germans did the same with the whole "honorary aryan" thing.
TLDR : Ask yourself, if you apply your belief, will the rest of the world follow? If not, who benefits from you applying your beliefs? Sometimes it is fine to have noble beliefs and realizing that they arent realistically applicable to the current world.