I have a similar story but inverse

I was the Emperor of Bizantyne Empire(I have used the 400yrs modification).
I was able to enforce my power with absolute crown authority after 30 years of accurate diplomacy,murdering and bribing,conquering the South Italy only with vassallization and reaching the Georgia and Armenia with wars..
But when my Emperor dies(at 77),then the disaster begun!
Basically the 70% of Empire declare war to the new Emperor!
After many years of long and fragmented fights I was not capable to resist to the alliance of count and dukes and I give up to my brother...Holy shit,he restored the Empire immediatly after I give up with only few dukes still in revolt and I become Prince,with the demesne of Byzantion and Thrake. Well,after 20 years I was capable to enlist a great army and to gain the alliance of HRE,marrying my son to the daughter of the Emperor of Germany.
I declare war to my brother and win the war after 6 years of fight...Byzantine Empire is mine again!
Honestly,the best strategy game I have ever played!And this is only the demo!