In this article, I outline processes that supported or hindered Elin, a mathematics teacher, to engage in pedagogy development. In a setting inspired by critical mathematics education, Elin was encouraged to bring societal themes into her upper secondary teaching so that mathematics was connected to social science subjects. A classroom environment was set up in which classroom discourses supported students' negotiations about their learning of mathematics. In this new pedagogical discourse, projects were introduced that while addressing the mandated mathematical topics of the curriculum, changed some key elements in how mathematics had been previously taught as well as the relationships between participants. Elin's narrated identities provided ways to understand shifts in Elin's ways of acting when gradually transforming her teaching. Elin's identities illuminated how she became aware of herself, her teaching organisation and her different ways of interacting with students. She identified and acted upon her perceptions of the new possibilities and different responsibilities that actors in mathematics classrooms have. Elin's fluctuating teacher identities reveal why she struggled at times and how she was constrained in becoming the teacher she wanted to become.