It absolutely is an ideology and it certainly has changed over time. There is no common unified continental European identity, and the founding fathers definition of White isn't the one you hold today. Germans weren't even considered White at the nation's founding, with Benjamin Franklin considering even Pennsylvanian Germans "Palatine Boors". "White culture" as we know it today didnt exist pre-WW2 in the same capacity it does now, and continental Europe has always been divided by nationality. Nobody spent billions socially engineering Whiteness, it was engineered through schooling, advertising, and mass media without a single dime being spent directly telling people they were White. It was politically convenient for an elite struggling with a rapidly expanding and diverse nation of various disperate groups to unite them all under the banner of Whiteness, in contrast to Black people. Just as the Capitalist West is an ideological construction designed to contrast against the Socialist East, Whiteness was designed to contrast against Blackness, the most clearly visible distinction amongst the American diaspora.
If you tried having this conversation with an Italian, German, or even Boer they'd laugh at you, because, just as there no material, ancestral root for Black people there is none for White people. Your ancestors weren't present here at the nation's founding, and if they were they'd still view you as a racial defective due to, as you stated, your own mutt genetics. Be grateful that you're not being herded into a concentration camp, segregated from the rest of society based on your lack of Anglo Saxon blood, or forced to slave away in a factory inhaling asbestos for 10 cents an hour.