Researchers have called for an approach that examines the conditions under which perfectionistic strivings (PS) may be beneficial or detrimental to psychological functioning. We adopted a self-regulation perspective and tested whether individual differences in self-regulation (procrastination, emotion regulation) moderate PS's relationships with achievement and well-being in an academic/work-related achievement context. A sample of 183 preservice teachers participated in a study that combined "traditional" longitudinal assessment (six performance situations over a 9-month period) with repeated ambulatory assessment (measuring well-being, procrastination, and emotion regulation during a total of 910 preparation phases that preceded performance situations). Mean levels of achievement, well-being, and emotion regulation were found to be stable over time, whereas procrastination decreased on average across preparation phases. Results from latent variable interaction models indicated that individual differences in intraindividual change in procrastination over time moderated the relationship between PS and well-being (but not achievement) in the expected direction: The less individuals decreased in procrastination over time, the more negative the relationship between PS and well-being was. Contrary to expectations, we found no evidence of a moderating effect of emotion regulation. The study contributes to a nuanced perspective on the adaptiveness of PS.