I don't like boasting about myself but through my experience in playing single player campaigns I learned this on the issue of secessions/civil wars:
In case you did everything you could ( and you ran out of money to spend it on securing loyalty ) to increase the loyalty of the rival parties to you and still didn't solve much of the problem, your last option is to let the secession or civil war happen but you should first check what are the provinces ruled by those rival parties ( This is something you should actually check in every turn even from the start of the campaign so that you're prepared beforehand for a secession or civil war ) . You can know that in the strategic map where it shows the political affiliations in each province. Once you know it, do this in order to reduce the difficulty of crushing the rebels and recovering the lost provinces: Give all the units of the rival parties' generals/admirals to your party's generals/admirals ( This is why I'd recommed having only 1 general or admiral of each rival party ). Then position at least one army in each province held by the rival parties. So when the secession or civil war takes place you immediatley proceed to recover the provinces before they raise big armies.
But there is a last option to check to see if you can still avoid the secession or civil war happens: Issuing the edict of +10 loyalty of the party that rules that province. This in several occasions saved me from a secession or a civil war.
And if it's very hard to appease a rival party because of features that are tough to handle: such as being pacifist, xenophobe, patriot, power thirsty, etc. You should consider killing the leader of that party so that the party features changes. Do it before the loyalty becomes very low because killing the leader of a party decreases loyalty by -5 ( I don't remember for how many turns ) and it could cause a secession instantly if you do it in the wrong time. But make sure the leader has a successor in order to avoid someone from your party switches to that party.
Extra things that helped me also to prevent secessions/civil wars is to choose these options of political intrigues that show up randomly:
- When the bandits offer you to intimidate the opposition: just reject the offer.
- When a banquet is going to be organized: just accept attending.
There are more random politcal intrigues I can't remember well by now but I'll find it out later to add them what are the options that avoid decreasing the rival parties' loyalty.