Help with Byzantium

Hey guys! I play Byzantium and I always get to a certain point, when the game is neither fun nor do I have a chance of surviving.

I start as Byzantium, I kind of stabilize my economy, have 700 regulars and 14 levies available. I take some Greek provinces like Euboäa and integrate provinces and raise control, also expand my Gold mine.

I attack Ottomans early and defeat them quite easily (which is too easy, they surrender basically everything when i defeated them few times, without conquering Bursa/ Prousa) and take all the Ottoman coastal provinces and tributes and like 100 Gold.

But after that, I am in death loop. My estates loyality is near constantly down, stability constantly -70 and I go bankrupt again and again. After the succesfull war against Ottomans, I have no more Levies, even if I release the old ones. And then the inevitable civil war starts and I have no levies and get crushed.

What can I do against that? Thanks!