What is willpower? Why does it feel limited? Why do some people seem to have more of it?
The answer, I think, is that much of our behavior is driven by reward seeking decisions calculated deep within the brain. The reward driven part of our mind is integral to action taking, where actions are defined broadly to encompass not only motor outputs, but also many mental actions, such as shifts in attention.
Motor behavior, such as reaching for an item, is reward mediated through the basal ganglia. Although the basal ganglia cannot directly control motor output, it can emit powerful activation signals to the cortex, which sends signals down to the muscles, converting them into actions. In the same way, dopaminergic structures around the VTA are able to activate cortical areas not for direct motor output, but for attention and thought. These subcortical structures use an algorithm similar to reinforcement learning to pursue reward.
Elsewhere in the brain, the prefrontal cortex and the temporal lobes, mediate the internal monologue and enforce broad cortical coalitions in directing cognition. Activity in that part of the brain can shift our attention to one task or another. It affects what we say and what we choose.
Willpower is a reflection on the connection between these cognitive processes, mediated by the prefrontal cortex and other cortical circuits, and reward seeking processes mediated by dopaminergic circuits. When we take action that is in accord with cognitive processes even in the face of competing short term reward, we say that willpower was applied. If the more immediate reward wins out even in the face of cognitive preference, then we say that willpower has failed.
Reward mediated circuits are not necessarily short-sighted. Although they have a (hyperbolic) future discount function, they do value future reward. And fortunately for them, cognitive processes are a good clue to knowing when future reward is likely. This is what happens when a person decides to cook their food rather than eat it raw. The cooking introduces a delay, but it improves the eventual reward. So cooking is a worthwhile task even though it delays the reward and introduces an effort cost. In this case, cortical knowledge successfully biases the reward seeking behavior. We might say that willpower has been applied. This especially applies the first time someone cooks food, before it becomes habituated.
The problem comes when cognitive cortical processes fail to act as reliable indicators of increased overall reward. If that happens, then reward seeking processes will come to discount cognitive intention when deciding what outputs to emit. That means that you might set an intention to study, but you will have many distracting thoughts about food or sports; activities which experience has told you are more likely to lead to reward than the studying.
Willpower is often thought of as limited. This is true because it interacts with a reward seeking process that learns over time how much weight to give to cognitive intention. If the reward circuitry learns to ignore the input of our intentions, then there will always be a struggle between cognitive processes and reward seeking processes — you might know that you should go to work, but find yourself playing video games anyway. But if the reward seeking processes learn that cognitive input is a reliable indicator of delayed future reward, then it will be relatively easy to apply willpower.
So, be mindful of the intentions you set. If you consistently push yourself to avoid immediate reward in pursuit of a delayed reward that never comes, you may find it harder and harder to overcome the allure of more certain pleasure.