How do you avoid getting ganged-up on in Europa Universalis IV? [closed]

We'd need a lot more details about your game scenario to give specific advice, but in general, the key is to maintain your own network of allies. Even a major power like France can't afford to go it alone in this game.

Some other things to think about:

• Be aware of who your enemy's friends and enemies are. Your alliance isn't going to be stable if your ally is also your enemy's best friend. Be sure to identify who everyone's rivals are.
• Are you taking people's land and creating aggressive expension? You've got to be careful here -- sometimes it's better to take vassels, or force them to release smaller nations out of their territory.
• When you do take land directly, take it from people who are already mad at you, like your rivals.
• Make sure you understand how manpower and attrition work. If you do get stuck in a disadvantageous war, fight (or avoid fights) defensively and let their invading armies suffer mass attrition on your land.

If you give some details about which nation you're playing, how far into the game you are, and what sort of wars you've been involved in so far, we might be able to give some more detailed advice.

EDIT:

One other tactic that can help if you really want to expand your territory without triggering a horrible backlash is to spread your wars out geographically.

For example, if you are Castille/Spain, you might fight one war against Granada -- none of the Christian nations will care. Then you might fight against france and take a province or two, none of the muslim nations will care. Then you might fight against someone in north africa. Then you might fight France again but this time just force them to cancel their vassals or release nations out of their territory. Then you might try a war in Italy. Later wars in France, you might take some territory on behalf of your vassal rather than yourself.

In this scenario, any of England, Portugal, and Austria would all be sensible major power allies (not that you can get them all to ally you at the same time). Combined with the smaller French nations you've been vassalizing, you ought to be able to stand up to France and whatever allies they've got.