Marrying an American and observing the massive cultural differences between our families convinced me of it.
> You absolutely are just as much of an American as anyone else living here.
Only if you’re using “american” tautologically to refer to “American citizenship.” And even then, the Constitution recognizes that naturalized citizens are less American than native-born ones, since we can’t run for President.
> America as a country is founded on the contributions of all of the people who moved here.
America is a country founded by British colonists. The country’s legal and political system, and most of its culture, comes from that founding population. Depending on the geography, there’s an overlay of German, Dutch, Italian, or Spanish culture. In specific areas, Americans that are part of the Jewish diaspora have had a significant influence. The influence of almost everyone else is localized or superficial. E.g. you can get Vietnamese or Thai food almost everywhere in the country, but the influence of Vietnamese or Thai language, culture, religion, etc., in American society at large is close to zero.
When people in my home country think of an “American” they think of people of British descent. They don’t think of Italian immigrants. Similarly, if you relocated the population of say Bangladesh to the US, and gave everyone US citizenship, they wouldn’t suddenly become “American.”
Culture matters, and a population’s culture is extremely durable: https://www.sup.org/books/title/?id=35594 (“In The Culture Transplant, Garett Jones documents the cultural foundations of cross-country income differences, showing that immigrants import cultural attitudes from their homelands—toward saving, toward trust, and toward the role of government—that persist for decades, and likely for centuries, in their new national homes. Full assimilation in a generation or two, Jones reports, is a myth.”). The American founders clearly recognized as much when they created a distinction between native-born citizens and naturalized citizens and wrote it into the Constitution.