Afghanistan is some of the most rocky and inhospitable terrain there is on the planet. The populace is some of the most irrepressible and hardy peoples on the planet. I doubt Americans - or most other western peoples - would put up with what they do.
The armies of all powers have been fighting with one hand tied behind their back. The population hasn't been rounded up and controlled. The locals are still trusted. Most cases where insurgents have been beaten has involved rounding up the entire populace and concentrating it and throttling the life out of the opposition. Be that the British vs the Boers or the Soviets against Latvians.
The number of soldiers present in Afghanistan in numbers compared to the size of the country is tiny.
The borders are utterly porous with fighters slipping in both directions when beneficial.

There might be several countries willing to help insurgents. However, most would also be rightfully concerned about what happens if the USA government wins. Getting supplies in would be problematic, considering the infrastructure the USA has to monitor borders.