Self-regulated learning can be conceptualized as the pursuit of learning goals by means of self-initiated control processes. Models of self-regulated learning postulate that goal-directed processes unfold within individuals from motivational states through volitional-control processes to goal achievement. Thus far, this hypothesis has mostly been tested with cross-sectional data, which cannot capture intraindividual processes. In contrast, the present study used a combination of intensive longitudinal questionnaire data and logfile data to capture intraindividual processes that lead to goal achievement. We tested medical students (N = 227) who prepared for a high-stakes exam on a digital learning platform. Students provided daily self-reports of their motivation and volitional control over 40 days. In line with our hypothesis, we found that volitional control functioned as a mediator between intraindividual changes in motivation and goal achievement. However, this was not true for all motivational constructs. While the effect of self-efficacy beliefs on goal achievement was fully mediated by volitional control, there were no substantial indirect effects for interest, expectancy of success, and value of success via volitional control. No indirect effects were observed based on interindividual differences. Our findings thus suggest that different motivational sources affect goal achievement in different ways, not all of which involve volitional control. The findings further highlight the importance of using intensive longitudinal data to examine the proposed intraindividual interdependencies in self-regulated learning.