IMO, Democrats consistently fail by being less progressive and not protecting the working/middle class. It’s neoliberalism that drives people away from Democrats.
Thoughtlessly enabling illegal immigration, which makes the country less safe, is also a problem. Immigration and providing asylum protections have to be done in a reasonable and proactive manner so as not to create instability in border communities, and to not alienate the people who live along the U.S. border.
Supporting foreign policies which embroil America in endless war is also problem, but it’s a beast with a thousand heads which sprouted way back when because of a policy of regional destabilization started by imperial Britain. The U.S. continued it with its absurd agenda against communist states and overthrowing of governments in LatAm. Unless all countries and leaders collectively call an end for it, it doesn’t stop.
But besides that countries with high happiness scores have strong social welfare programs and healthcare. Corporations need to be taxed more and be held accountable for their use or abuse of natural resources. In short, America needs to utilize the Nordic model for national and international policy making. In fact, I think it’s useful for all countries.