I think the issue with a national identity is it cannot be forced. Pre-World War II, I think most Americans related to their state as much as being an American, but WWII, the Cold War, and easier transportation across the US, and population migration is what got us thinking Americans first state second. But that all happened pretty much organically. The EU seems to be trying to force an identity and not shockingly there has been a ton of backlash.
As I noted, there is also a lot of it happening organically. The backlash is for the most part people projecting their diffuse frustrations with various aspects of their lives on the EU and a perceived identity policy. At the same time, there are people voluntarily and happily going from one EU country to another to study and work, and love it.