Reflecting on teaching situations is important for student teachers’ professionalization and can be understood as a learning opportunity. However, despite being important for their professional development, only little is known about the interindividual psychological processes that occur when student teachers reflect on their teaching. The present study examines 146 student teachers’ teaching-related self-efficacy beliefs at two timepoints (at the beginning and end of one semester) and their emotion word use in 239 written reflections on differently perceived teaching situations (135 mastery and 104 challenging) using Wald tests and path modelling. First, our results show that student teachers use more positive than negative emotion words in their written reflections. Second, examining relations between situation and emotions, we show that emotion word use differs significantly between teaching situations. Further, higher negative emotion word use in reflections on mastery experiences (positive teaching situations) is associated with lower self-efficacy at the end of one semester. Thus, it is important to focus particularly on the challenges that student teachers experience during teaching when aiming to promote their self-efficacy beliefs.