Nationalism has but one meaning: You are born in a certain place, part of a certain culture and heritage and history, all of which constitutes an existential-metaphysical substance and a "lineage of ideas". The culture/society plays a central role in forming the individual. The point is that your country is YOUR country, and you should work to make it more after your own image, to make it better however that means to you.
Instead, most countries on the planet currently are so fucked up with fascism and petty power games that it's nearly impossible for most people to participate in making their country better. So instead they go to another country and pretend like that is their country... this is of course a tacit admission that their home country is pure shit.
Nonetheless, going to another country and pretending it is yours, doesn't make it so. But there is still justification to leave a shitty place one was born for a better place, just be honest about what you're doing and why. Saying that Mexican immigrants to the US now own the US, whereas natural citizens of the US don't own it anymore, is seriously deluded. Just admit why you are really coming here: "Hola, I would like to say that I love the United States, because my home country is shit. I would like to participate in your culture and idea-heritage instead."