Emerging research documents the self-control consequences of individuals’ theories regarding the limited nature of willpower, such that unlimited theorists consistently demonstrate greater self-control than limited theorists. The purpose of the present research was to build upon prior work on self-validation and perceptions of mental fatigue to demonstrate when self-control is actually impaired by endorsing an unlimited theory and—conversely—enhanced by endorsing a limited theory. Four experiments show that fluency reinforces the documented effects of individuals’ willpower theories on self-control, while disfluency reverses the documented effects of individuals’ willpower theories on self-control. Moreover, these effects are driven by differential perceptions of mental fatigue—perceptions altered by individuals’ level of confidence in their willpower theory—and are bound by conditions that promote effortful thought. Collectively, these findings point to the malleable efficacy of willpower theories and the importance of belief confidence in dictating this malleability and in modulating subsequent self-control behavior.