Half of America's major political parties disagree with America First in any sense of the term. Look at how the democrats go nuts at literally any policy that promotes home grown fossil fuels production, for example.

Because they disagree that it is in America's interest to promote home grown fossil fuels production. They believe that climate change poses vast threats to U.S. interests, that the only way to arrest climate change is through global coordinated action, and that the only way to accomplish that global coordinated action is by the U.S. taking the lead in reducing fossil fuel production.

That's entirely a "Let's advance American interests" policy. I happen to disagree with it (I don't think the second and third steps are plausible), but it's genuinely motivated by strategy to pursue what they believe is the best path to protect American interests.

What it means is that we're no longer going to willingly bankroll the lifestyles of our partners to the detriment of quality of life or America's national security posture for the simple reason that we can't afford it any longer.

We could "afford" to continue our present tariff structure with Canada going forward. We can "afford" to continue to not own Greenland. The changes that Trump is proposing are choices, not necessities - and there's very good arguments against nearly all of them. I don't expect you to find the arguments persuasive (for example, that it's actually better for U.S. security for a western Europe filled with historically hostile and warring countries have smaller national armies and the U.S. to be in charge of most of the heavy military forces there, rather than the other way around). Simply to recognize that rejecting Trump's vision of foreign policy doesn't mean putting America's interest "second," or whatever the flip side is to "America First."

The status quo in so many areas has been warped to a point so as to not be sustainable. Do you think America can keep footing the bill for defending all of Europe and Canada, all by ourselves? Forever?

Do you really believe that was true? European defense spending in 2015 was approximately $328 billion. Other inarguable members of "team West" - Canada, South Korea, Australia, Japan, Israel - spent another $120 billion. For a combined total of about $450 billion. U.S. defense spending that year was about $596 billion. Hardly "all by ourselves." U.S. spending on defense in 2015 was about 3.3% of GDP, which is basically a single percentage point above the global average of 2.2%. Again, hardly anything unsustainable - and a position that comes with lots of national security perquisites, since he who pays the piper calls the tune.

sipri.org: PDF document