It was difficult for the American colonists to make the initial break from England because many Americans considered themselves Englishmen, they did not believe they could defeat the British military, and they feared losing England's protection.
Most of the early Americans were only a generation or two removed from living in England; most of them still had English relatives. Consequently, it was difficult for many Americans to imagine fighting against their home country. It would be somewhat similar to an Alaskan contemplating going to war against the contiguous United States.
Additionally, England possessed the world's premier military during the 18th century. On the one hand, many Americans did not believe they could defeat the British military which had famously defeated the Spanish Armada and won the Seven Years' War (known as the French and Indian War in the United States). On the other hand, many Americans feared losing the protection of the British army and navy. Without status as British colonies, the Americans could have been vulnerable to attacks from other European powers who wanted to re-establish a presence in the New World.