That's an unrealistic ask. 40 years have passed since 1975, almost two generations. People die, the will of the country changes, and you cannot run elections on a single issue. Even europhile countries like France and Italy have periodic bouts of euro-skepticism, and places like Texas, Quebec or the Balkans demonstrate that tribalism is always lurking anyway.

I think the EU should simply improve its outreach programs. Some of them were very successful (Erasmus), some weren't (the EU flag on buildings is almost imperialistic).

The truth is that getting 511m+ people to agree on things, when they won't even speak the same language, will always be a huge challenge. Unfortunately, there is no alternative: this planet has to learn to resolve its problems by talking and agreeing a common course of action, so might as well start with your immediate neighbours.