The only thing I can think of is you have to feed way too many soldiers, so their salary is dragging you down.

Before that happens you can use only peasants for garrison - they are very cost effective. You only need enough of them to keep the order 75%+. Then you only need 1 or 2 serious army for invasion early on.

If you expand fast you should be able to support yourself with more population and tax. Many players report they went after the Greeks on Balkan for more cash.