Self-reflection is often viewed positively; paradoxically, however, it is also associated with distress, potentially because of its relationship with rumination. Focusing self-reflection on positive themes may be one way to promote adaptive self-reflection. This study examined whether the disposition to engage in self-reflection motivates use of a journal containing positively focused writing prompts and moderates the benefit gained from it, specifically when rumination is controlled for. For 28 days, participants (N = 152) accessed an app-based mental health intervention containing various features, including the aforementioned journal. Outcomes of self-regulation and psychological wellbeing were assessed, controlling for time spent using other app features. As expected, journaling was associated with improvements in psychological wellbeing but only when baseline self-reflection was average or higher. Journaling was also initially associated with improvements in self-regulation, but this was diminished after controlling for time spent using other app features. Findings suggest self-reflection could be a strength for fostering wellbeing when it is directed in a positive way.