Looking over Obama's proposed 35K troop increase, I'm honestly not sure quite what to think.
On the one hand, thinking about the past is the classic sunk-cost fallacy: whatever time and effort we've put into Afghanistan/Pakistan is irrelevant at this point, and what matters is what makes the most sense right now and going forward.
On the other hand, I can't just dismiss our involvement to date. When we went into Afghanistan 8 years ago, we changed a lot of stuff, and integrated American presence into the every day lives of the country and it's people. If leaving means that many people's lives will change for the worse, and if our continued presence can prevent that (two very big if's) then I'm very uncomfortable with the idea of just wiping our hands and saying "see ya!" Of course, there's a temporal aspect to that as well: if our presence needs to continue indefinitely (which realistically, it can't) then it's a matter of having bad stuff happen now, or later. And since there's additional cost to us (both money and lives) the longer we stay, that points to a quicker exit. But nobody can say with certainty that there's absolutely nothing we can do.
In other words, we need to value an indeterminate number of American lives and American money against an interderminate number of Afghani lives and Afghani money for an indeterminate number of years. Argh. I do not envy Obama this decision one bit.