ready to give it a try

[QuintusSertorius]

i dont think your Culture can decline , i havent had that happen , it can be very hard to get it up it takes a long time , just keep building your colony crap where you can , it come up every 16 turns or so .

Your culture can decline if you're close to a region with a different culture and your governor doesn't have very high influence. More likely, though, where a building is present which converts to something else. The Native Garrison is the classic one, anywhere from Anatolia eastwards it converts to Eastern Imperial.