Girls and women are disproportionately affected by delayed diagnoses of attention-deficit/hyperactivity disorder (ADHD), but research on the impact of this is limited. Our study aimed to centre lived experiences of women with late-diagnosed ADHD to increase understanding of the impact of such delays. We employed a mixed methods survey to investigate the perspectives of 28 women with late-diagnosed ADHD. Results starkly demonstrate the criticism and lack of support participants faced from society and medical professionals, illustrating the negative consequences of delayed ADHD diagnosis on quality of life and mental health. Participants commonly reported internalising criticism and described disconcertingly low self-esteem; citing guilt, shame, and negative self-perception due to delayed diagnoses. Participants found diagnosis revelatory, their lives finally making sense; citing healing, improved self-esteem, and life feeling more worth living. The adversities faced from delayed diagnoses were described from early childhood, through adolescence, and adulthood. Participants reflected on ‘what could have been’, and described grieving the lives they could have led if diagnosed earlier. The negative impacts of missed ADHD diagnosis are broad and span life stages. With potential implications for public health policy, this work highlights the importance of increasing girls’ and women’s access to ADHD diagnoses to address avoidable detrimental hardship.