You're not using patch 5 so we will probably mess up your save. You have to not use a general for battles if he has anything more than 1 in ambition. High gravitas characters you have to get rid of - I'd say yours and the enemy, depending on balance. You have to stop conquering new lands as that seems to be part of the trigger as well. But that's only temporary if you've gotten to that point it's probably beyond reversing.