Part of the problem was that the American colonists were also doing things that made the colonies far more expensive to run.

The North American front in the Seven Years War was relatively major, costing the British a lot more than just fighting in Europe. The Dominions of the United Kingdom also started looking for independence after they were asked to participate in World War I.

You also had American colonists fragrantly violating the treaties that the British made with American Indian tribes regarding borders. A poor farmer in the Revolution might be fighting for the right to settle in the Ohio Valley or Kentucky.