In recent years, the wellbeing of students and staff in Higher Education has received increased attention in pedagogical research. However, the experiences of Graduate Teaching Assistants (GTAs), or PhD researchers who teach alongside their doctoral studies, are notably absent from this literature. In this mixed-methods study, we examined predictors of GTA's wellbeing in UK institutions. This demonstrated that greater perceived agency and lower levels of over-commitment predicted higher wellbeing. Other variables, including social identity, role pride, work/life balance, effort-reward imbalance, and perceived effectiveness of teaching, did not predict wellbeing. We also qualitatively examined the identity management practices of GTAs, using a brief story completion task. A reflexive thematic analysis generated two dominant themes (1), the 'paradox of credibility', whereby GTAs reported grappling with a concern to be perceived as credible and worthy of teaching whilst also managing student expectations about their own knowledge, and (2) GTAs' desire to use their staff-student role to engage in 'approachability and advocacy' with and for students. We end with a set of recommendations for Higher Education Institutions that employ GTAs, informed by first-hand accounts of lived experiences.