While motivation has been recognized as a key catalyst for the uptake of self-initiated professional development (PD), motivational changes that occur in the course of self-initiated PD practices remain unexplored in English Language Teaching (ELT) contexts and beyond. Yet this research area warrants closer attention since motivation is a dynamic construct displaying nonlinear trajectories over multiple timescales. The exploratory single-case study reported here aims to tackle changes in motivation and the sources of these changes as reported by an English-as-a-foreign-language (EFL) teacher trainee who engaged in ELT assistantship in a real classroom as a self-driven PD activity over four months in Turkey. Drawing on data from self-ratings of perceived motivational intensity, journal entries, and semi-structured interviews, the analyses showed that the focal participant's motivational development manifested a constantly evolving pattern with temporal ups and downs, fluctuating in response to a complex array of individual and contextual factors such as shifting self-efficacy beliefs, display of coping efforts, perceived success/failure of the lessons, and group dynamics in the classroom. Implications for facilitating (trainee) teachers' sustainable engagement in self-directed learning are discussed.