"So why oh why can't I seem to stick to [my resolution]?"
At each moment in time, a new "you" comes into existence. Over short time scales, the new "you" is indistinquishable from the "you" of five seconds ago. However, the younow is quite different from younextyear.
If a fairy godmother offered you a choice between a bag of Doritos now, and losing 30 lbs immediately, of course younow would choose the weight loss.
But instead younow is offered a choice between the immediate, certain pleasure of the Doritos in exchange for a slight loss of weight in younextyear. Why should younow make such a sacrifice for younextyear? Younextyear might not even exist! What benefit does younow get from younextyear's increased babe magnetism? Nothing! Well, maybe a little smug self-righteousness, but how can that compare to the crunchy, salty goodness of Doritos?
Plus, even if younow cooperates, younextyear's svelte figure depends not only on younow but on youtomorrow, youtomorrow+1, etc. What guarantee does younow have that those guys will cooperate? Given how youlastyear behaved, younow knows the odds aren't good. So why should younow be the one to suffer?
Moreover, our monkey ancestors probably benefited from lack of impulse control. Fat and sugar only became abundant in the last 100 years or so. Proto-humans didn't have to worry about keeping slim -- that was the easy part. The hard part was not starving to death. Until about a 100 years ago, you would've been stupid to turn down extra calories.
That said, our brains have evolved some crude mechanisms for impulse control, to integrate all of your once and future yous into a single You. (After all, it would've been stupid for protohumans to make bake their seed corn into biscuits.) But I suspect that our impulse regulators are a relatively recent evolutionary development--I don't think they work very well. Especially when it comes to food.