Occupational stress has been identified as one of the biggest contributors to therapists’ poor mental well-being; however, these ‘wear and tear’ effects are not universal. This study aimed to distinguish between these experiences, by exploring potential avenues through which a therapist’s professional life may impact their well-being. Previous research has highlighted four avenues: self-doubt, developmental depletion, developmental growth, and years of experience. A sample of 65 cognitive behavioural therapists were recruited from four Improving Access to Psychological Therapies (IAPT) services and a cognitive behavioural therapy training course at Buckinghamshire New University. A quantitative cross-sectional survey design was employed. Each participant completed a questionnaire consisting of subscales of the ‘Development of Psychotherapist Common Core Questionnaire’ to measure potential avenues (Orlinsky et al., 1999a), and the Warwick-Edinburgh Mental Well-being Scale to measure mental well-being (Tennant et al., 2007). A multiple regression and post-hoc mediation analyses were conducted. The multiple regression analysis found three significant direct pathways predicting well-being: self-doubt, developmental depletion, and developmental growth. The post-hoc mediation analyses found two significant indirect pathways between self-doubt and well-being mediated by developmental experience (growth and depletion). Years of experience had a non-significant direct and indirect pathway predicting well-being. The results suggested that therapists experiencing more growth, less depletion and less self-doubt experienced more positive mental well-being than those experiencing depletion and self-doubt, and less growth. When therapists experience high self-doubt, whether they also experience depletion or growth, they experience poor mental well-being. The implications for practice and future avenues of research are discussed.