I bought this game a few days ago and have a few questions.
Castille and France are currently steam rolling the globe. I'm England in the year 1593 and haven't conquered Scotland. (I tried earlier but had to deal with 25 regiments of castillian soldiers marching around my colonies and Ireland)
How do you stop these mega countries? Will they implode by themselves? Is centralization a good thing?
Centralization is a very good thing.
As for mega countries, you basically need to beat them in a single war, and you can make them release fairly big areas (aka, like 5 of the countries they have absorbed). They will never implode (well, they have never imploded in any game that I have played at least without my personal intervention), and you basically need to personally beat them in war after war to make them smaller.
As England you are blessed with a great navy, one that basically allows you to win any battle involving the British isles, you just need to smash the navies of countries that are fighting you, and they will basically never be able to fight you evenly on the ocean again (since building up a world class navy again takes like 2000 money and a few decades at the very minimum even if you have the money to afford it, and an AI almost certainly won't be able to build up again).
The proper thing to do VS Castille invading you, is just smash their navy, and they won't be able to land any troops to attack you (except for maybe the first wave), and after that they won't ever be a real threat against you in the isles again.