Why was it difficult for George Washington to form and keep a large army?
He didn't command a professional full-time Army, the troops were not enlisted for long terms of service, food and materiel were short, cooperation from the Continental Congress was slow, and he was up against the largest and best-trained Army in the world. Many of his soldiers were farmers and wanted to return home in spring to plant crops and fall to harvest crops.
George Washington was a General of the Continental Army in the 18th century, the Continental Congress later elected George Washington as the Commander-In-Chief. Making him "General George Washington Commander-In-Chief of the Continental Army" he held this position through most of the American Revolution.