-Build buildings that help control public unrest like temples, amphitheatre and the such.
- move your capital to the centre of your empire
- Exterminate newly conquerend provinces, it will make it easier to control them later on.
- Destroy any religious building of another faction.
- Don't build the farm upgrades, it will make population grow too fast: the more population in your settlements, the more difficult to control them.
- garrison you cities: use cheap units for this purpose, for garrisoning it's the quantity of the troops, not the quality that will increase public order
- Conquer the whole of Spain and make sure you have ports in every one of these settlements, they will give your income a nice boost.
- get Massilia back
- if necessary give up on your northern provinces, just make sure you destroy every single building in it. Leave them, with a bit of luck there will be rebels in them, forcing the Britons to siege those cities and lose some troops by conquering them - afterwards retake them and exterminate the population.

You have a big empire, you won't lose in a few turns, there's still plenty of time to recover. Besides, the most intresting and joyfull campaigns are the difficult ones

Good luck