Just to add to what I/others have already said, another thing that you may find useful to do in times of an economic downturn is set up a colony in richer areas.

As the Scipii I typically end up with a poor economy after the defeat of Carthage and use this method to gain new wealthier lands. This is expensive at first, but exceedingly profitable later on.

Typically, I go to the town of Kydonia on Crete and, from there, use that as an outpost into Asia Minor (Turkey). You do often end up having to finance massive armies to control these regions and keep the enemy (usually Egypt) at bay, but the wealth in the area is large enough to be capable of supporting them.