I'm playing as France and I'm getting war declared on me by a super power every so often, which gets England, Spain and Portugal involved against me usually. My only hope is to usually stack troops on a mountain and defend and just hope they keep attacking that mountain defense. What should I do to keep them from declaring war on me all the time? I just want to live and conquer in peace!
I just want to live and conquer in peace!
And therein lies the problem :) As France, it's not very difficult to be universally hated, due to its central position and multiple powerful neighbours. You have probably accumulated too much aggressive expansion(AE) from conquering left and right (particularly provinces these superpowers consider part of their rightful domain) and everybody's outraged or hostile. There's a reason why AE is sometimes referred to as "badboy" :-)
It happens. What you need to do is find powerful allies to keep an armed peace, not unlike the Cold War. Hungary and Poland/Commonwealth are good choices, the latter mostly useful against Austria. Some of the Scandinavian powers can help with England's fleet. Larger HRE princes that do not border you can also be of assistance -- Savoy, Brandenburg, Bohemia are typically strong contenders. Generally, you need to be looking for allies among coalition's bitter enemies. Declare some of the stronger coalition members as your rivals (they hate you anyway), and placate, bribe and befriend their enemies into allying with you. Doesn't matter if they are not very powerful by themselves, they have friends of their own that might help. And for all that is holy, don't participate in wars on the aggressor side until tensions are eased.
The take-home lesson is to be careful when expanding. Don't take too much provinces in one go, prefer vassalizing and releasing small nations, and always check how much AE your current peace offer will cause. It takes a long time before coalitions dissolve, typically more than the duration of a truce, so it's preferable to take small bits at a time to keep everybody happy.
I personally didn't play as France until now but here are a couple of general rules of thumb that'll hopefully work.
Trying to get one or a couple of them as your allies could be beneficial. You'll reduce the number of super power threats and also have their support at wars against others. If they are also enemies with each other (England/Great Britain and Castille/Spain probably will be), you can have extra enemy of enemy relation bonus with one by marking other as your rival.
Improving relations with them also could prevent or delay wars for some time. You can try having royal marriages, guaranteeing independence or gifting money to keep relations good.
Alternatively, you can declare war on them immediately after/during, they fight long wars with other countries which consumes most of their manpower. They will be weaker and more vulnurable. If you can steal some provinces or make them release nations, you'll weaken them more. And after the war you'll have some time to get things in order until the truce ends.
Currently, I'm playing as Austria and constantly declaring war on France after the previous truce ends just to keep them weak. I also had Spain, Brandenburg and Poland as my allies. When they tried to declare war back on me, they had a huge pile of armies coming from every direction.