One reason we're not going to win in Afghanistan is that, even if we could, it wouldn't matter much to the destiny of this failed state.
Afghanistan's troubles are myriad. They're tribal, ethnic, national, regional, even international. Like it or not, Afghanistan is a football that's being booted about by the United States, NATO, China, Russia, India, Iran and, of course, Pakistan. In this context the Taliban and Islamist extremists are mere chump change.
India is playing Afghanistan to get at Pakistan. China is playing Pakistan and Afghanistan to get at India. Russia is playing Afghanistan to get at the United States.
If you want to get a sense of the confusion and contradiction in the Great Game now being played out in Afghanistan, read this article from Asia Times Online: