
Hello good sir. By no means I am an expert, but I shall do my best to give an appropiate response to your problems and help you find some solutions.
I'm relatively new to this game. When I bought it I thought it would be amazing to play, but it turns out like this;
Lots of barbarian uprisings and lots of civil wars. I have played as the Romans (Gave up due to a never ending cycle of civil wars because of the Populists) and the Pictii (Gave up after rediculous amounts of barbarian uprisings) both on normal difficulty settings. I want to be concentrating on the politics, domestic and internationl. But, I end up having to use all my money on building a military to fend off the afore-mentioned barbarians and also when I have re-strengthen my military after a civil war, which leaves me no money to do any proper politics. And not to mention having to set up every trade route after every bloody civil war! It's rediculously tedious, and so spoils the game for me.
As Cheexsta already mentioned, the Pictii are one of the hardest tribes, matched by the Suebi (and perhaps the Daci as well?) due to their isolated starting position. After all, one of the direct problems, aside from an ever so dangerous threat of barbarians, is that there are no civilized tribes or civilizations nearby to be conquered.
This is turn dramatically reduces the speed at which your own civilization increases, needed for the ability to colonize around your home province. In the case of the Pictii, the only real hope of getting anywhere, and the same can be said for the other two tribes starting on the island as well, is to conquer the Gallic tribes and the Belgae tribes.
Of course this will be, in case of the Pictii, a difficult thing to achieve since the economy is already quite bad. So instead, I'd say you should focus on getting a stable income first. Make trade agreements, create a trade route (Preferably with a nation that you won't conquer or declare war to in the next x year or so). With a stable income, you can attempt to conquer the other tribes on the island and then continue to go to the mainland. I dareth wager that after a few years you will be a somewhat bigger power than in the beginning, though by no means a colonial super power that has conquered most of the world yet. :rofl:
The Romani are quite simply, only matched by the other great kingdoms or empires. The Macedonians, the Seleucids, the Egyptians, the Carthaginians, they are all quite easy as you start with a stable economy and, in most cases, a pretty decent army. More on this in the next part.
If anyone can tell me how to stave off countless civil wars, and countless barbarian uprisings that interrupt the game so that I can play the game of politics and warfare with other nations instead of myself, I would be most happy. I just want to enjoy the game, but I can't.
Alright, now let us move on to the politics. You have problems with civil wars in a Republic because of the populist? Try to prevent that. Of course as you're reading this you're probably thinking "I tried that already, because else I wouldn't be here", but don't bite my head off yet.

(I really need to stop using emoticons... blegh)
In a Republic, there can be various reasons for populists to gain power. I myself most of the time end up having either the civil or the military faction as the strongest with populists being to reduced to something under the 5. How? Quite easy.
Always, at least once a year, look at the unemployed list. You can give people titles. Some people will be carrying empty squares. This means they want a title and if they do not get the aforementioned title, their love for the populist faction will increase on a steady rate. However, don't just give them any title you have just yet.
In order to increase other factions and reduce the populists, you must appease individuals. So if someone wants to become a Tribune of the Soldiers, make that person a Tribune of the Soldiers. After all, that'll make him join the Military faction instead of the populists. Do this with everyone. First give out the titles you to people that have specific requests you are certain you can fulfill, then get rid of the empty squares if any are left, by all means possible, and afterwards just try to give everybody a title, even if it isn't what they wanted (After all, you shouldn't just turn anybody in a censor, right?)
And, of course, there is always another option available for you. If you get tired of playing with a Republic, then try out Macedonia, the Seleucid Empire or even Egypt. They all have monarchies and whilst there is always a small chance that some counsellors will back another one of your heirs instead of the one chosen, it requires less attention in general than a republic with their bloody populists.
Also, regarding the barbarian problem. It is basically impossible to effectively prevent any form of threat from wandering barbarian hordes. However, the closest thing you can do is simply go to each province once in a while, wait until the tribes attack, and destroy them, reducing the barbarian power to 1 for some time.
Hope this helps and if it doesn't, please let me know so I can try to find some better advice. Good luck with the game.
With friendly regards,
Julianos Valent(inus)