Nationality is a very large factor in the latter stages of the game, the top priority should be building your culture in those cities so artists and actually just building culture are a good idea. Large garrisons don't help very much if the city is Worsening (citizens resent the foreign oppressors), they're much more useful once the city flips to improving.
Beyond this, either crush the final Roman city (which will greatly alleviate your situation) or avoid war with the Romans, being at war with the dominant culture in a city aggravates an already difficult situation.
Remember you can also bribe cities to buy you some time, if you have enough money you can keep them from revolting forever. Check out the button at the bottom of the Rev Watch popup.
Another backdoor way to improve the situation is to let the Romans capture the city, either by setting them free or intentionally pulling out defenders. When you recapture the city, the show of force essentially resets the rebelliousness of the population ...
Holding on to foreign territories is definitely not easy in this mod, but it is entirely possible to do so ... it takes some planning and effort.
