“Our” civilization, “our” country in a uniquely American sense. The people who use these terms mean the America that existed before the Civil Rights era — an America in which being white (like a marshmellow?) was enough to ensure some modicum of economic success. If one was white, one had a leg up (a very long leg, at that) on the job at the local factory/mill/plant where it didn’t really take smarts to earn okay money. After the passage of civil rights laws barring workplace discrimination, these folks had a lot more people to compete with for the limited number of jobs. If the job in question could be done by anyone — if having a deficit of melatonin wasn’t a requirement — the extra people in the job market drove down wages. That wasn’t how “their culture/civilization/America” was supposed to operate.
Then there was integration of the teaching staff in public schools, and horror of horrors, some non-white teacher might paddle their precious little snowflake. When the student-bodies were integrated, “what if — you know — ‘one of them’ asks little Mary to the Prom?”
Seen from the perspective of someone with a fairly typical American view of racial propriety, this is exactly what “losing our culture/civilization/country” looks like. Why wouldn’t they use these terms to decry the changes they were (and still are) powerless to stop? That is what terms like “make America great again”, that seem like total gibberish to progressives, are so successful in motivating so many people to vote for a charlatan like Donald Trump. “Make America great again” entails changing the system back to one in which they perceive that they have the power to direct the trajectory of their own lives and the lives of their children, grandchildren, et cetera.