Why was the task of governing such a difficult one for washington and congress?

Probably because, first of all, the states still didn't act like a union during the time, and had many contrasting opinions of the government. Also, it was a bit difficult to form a governing system that isn't like the monarchy that didn't allowing the states to hold more power than the government, so the Articles of Confederation was made. Of course, there were many conflicts with that and there were quite a few rebellions that took place, so the Constitutional Convention was birthed. The Convention was used to evaluate the advantages and disadvantages of the system that sparked from the Articles of Confederation. There was also the slight economical problem after the American Revolution, and they ended up taxing the citizens for their alcohol, which was basically why the revolted anyways. So the Whiskey Rebellion also caused some problems. Governing's difficult when you're trying to keep the land free but also controlled, so yeah.

Or, you know, it could also be something as simple as how Washington hadn't even wanted the job anyways.