You need more cities! (sorry but I couldn't resist it.) Seriously though, you do! There can be a painful period with Republic, as others have already said, but you also need to play a little differently. For example, many people like to have spears in every town. You can't afford this and, more to the point, you don't need to do this. Anyway, who's afraid of spears? In republic you build an offensive army and go and take some more cities to increase your income and reduce your unit costs. Another point is that roads are so important. You will not notice the commerce bonus in republic if you haven't got roads on the tiles that your citizens are working. How many workers have you got with your 15 cities? How about you post a game here and someone can then tell you why republic is not working for you (and I can stop guessing what the problem is).