Reflection plays an important role in medical students' ability to develop diagnostic competence through practice with clinical cases. However, it is not easy for students to develop expert-like performance through self-reflection alone; conversely, seeking feedback from experts constantly in practice is impractical. This study investigates the design and effects of computer-based cognitive feedback in practice-oriented learning in an online system. The system allows learners to work with simulated cases and self-review and reflect on their diagnostic processes that the system captures visually. Moreover, the system provides learners with feedback about the gap between their performance and expert performance on a set of key components of the diagnostic task, i.e., selecting clinical examinations, making intermediate judgements, and reaching diagnostic conclusions. The findings show that cognitive feedback on task performance can reduce learners' anxiety and frustration while working with complex tasks. Moreover, by providing feedback on learners' performance on a set of key components of the task, the proposed approach has shown promising effects on improving learners' diagnostic performance. Compared with its effects on learners' diagnostic conclusions, the approach is more effective in enhancing learners' performance when selecting clinical examinations and making intermediate judgements, both of which may improve learners' understanding of the mechanism underlying the diagnostic process.