1) Check your infamy (burned flag at the upper left). NEVER - and I mean NEVER EVER - go above your infamy limit. Try to stay below 15 unless necessary, because infamy will reduce relation to ALL countries over time. Having 20 infamy for 25 years means almost everyone will have relations <0 towards you.
2) Keep your stability up. Try to stay at +3 stab at all times. High stab will reduce revolt risk, while negative stab increases RR.
3) Conquered provinces have nationalism. They can - and will - revolt until they have become one of your cores, that means 50 years. The effect wears of over time, so the first ten years are the hardest.
4) Keep WE (war exhaustion) low! If you use large stacks in enemy territory (>2-5k during the early game), you will suffer "attrition", meaning soldiers die from cold, illness or simply desert. Attrition triggers once you exceed the supply limit of a province. If you enter a province with too many men or stay until the end of the month, you will suffer attrition. Usual maximum is 5% (meaning 5% of your soldiers above the province supply limit will die when you enter it or the month changes), but during winter, esp. severe ones (Russia, Scandinavia) it can go up to 15%. This attrition will usually be replaced immideatly, so you will not notice any change in the number of soldiers on the field. At best you will notice your manpower pool depletes slowly.
BEWARE! Avoid attrition at all costs! Whenever attrition occours, your war exhaustion increases. War exhaustion increases (not only, but esp.) your revolt risk and will take years to fade away. Don't use 10.000 men to siege a lvl 1 fort if 2.000 can do the job, don't run with 50k stacks after 5k enemies.
5) Check if other countries have claims. A claim is if one of your provinces is considered to be a core province of another nation. If you took provinces from England, they will still have cores on them, unless you force them to revoke the claims via peace treaty or the province stays out of their hands for 50 (100?) years.
The AI is eager to DOW (= declare war on) any country it has claims on. I noticed this as Holland - I was a big friend of ENG - until I got hold of Calai, which they considered to be a core. They left my alliance, warned me, embargoed me and then started a war.