A thoughtful post. Although I do believe I can come up with at least two counter-examples to your thesis: I think the Brits defeated a guerilla insurgency in Malaya when they ran the place. And we defeated one in the Phillipines when we took them over. The situation in Iraq is further complicated by the fact that there are several conflicts going on simultaneously:

Sunni vs Shia
Baathists vs everybody else
Kurds vs Arabs
Government vs militias
militias vs militias
tribe vs tribe
and of course, assorted participation from Al-Quaeda, Syria, Iran, and if SDAI-Tech1 is to be believed, the French.

The only thing all these groups have in common is they all hate us, except maybe the Kurds, and everyone hates them. It's what the military calls "a target-rich environment", which is a polite way of saying "we're surrounded".