How'd you colonize all that land so quickly?
Native colonization (native idea) will give you 1 colonist from the start, -100% Admin Technology cost (native idea) lets you reach Admin Tech 4 in a fair amount of time (about 1500 each tech level), and then you'll be able to buy the full Exploration idea tree with the Diplo points you saved. So that's 3 colonists, and the economic power to mantain 5 colonies growing at all times comfortably (and I still had a huge army and fleet, and +3 advisors, at least before reforming my government which removes all the wonderful native buildings and bonuses). Unless the Europeans are extremely quick in their colonization, and they usually aren't, you can safely shut off your continent.
Honestly, I think the best country in the world if you want to colonize North America quickly and without opposition is, by far, a Native American tribe right now