Introduction: Non-suicidal self-injury (NSSI) has disseminated dramatically especially in young persons with mental health vulnerability. Therapeutic online interventions have been tested, though little is known about how they are experienced by users. Specific treatment for NSSI is needed, and Internet-delivered Emotion Regulation Individual Therapy for Adolescents, IERITA has been assessed feasible in child and adolescent out-patient services. Aim: In this study, we aimed to explore the experience of IERITA in addition to treatment as usual in NSSI engaging adolescent out-patients and their parents. Methods: A qualitative study nested within a controlled feasibility trial. In the period November 2021 to February 2022, we conducted five synchronous, online semi-structured focus group interviews (via Microsoft® Teams): three with adolescent participants aged 13-17 years (n=9) and two with parents (n=8); all provided IERITA in a 11-week period. Interviews took 1½-2 hours and were analysed using thematic analysis. Results: Three main themes were generated: Theme 1) “Be forgiving”: Exhaustion and the consequences of fatigue. This encapsulates experiences of exhaustion and fatigue including consequences for the therapy processes, before, during and at the end of intervention. Theme 2) inter- and intrapersonal insights as facilitators of change. The framework of emotion regulation facilitated a focus on underlaying issues, rather than solely on NSSI, leading to an understanding by a shared language. Theme 3) internet-based intervention: writing to the therapist. Participants highlighted contrasts and similarities between face-to-face and internet-based treatment, focusing on the therapeutic alliance, a lessened interaction-pressure, the tempo of writing leaving room for reflection, and the flexibility of online-delivered therapy. Conclusion: Fatigue due to the therapeutic engagement and previous help-seeking processes created barriers for engagement in IERITA. Emotion Regulation Therapy is experienced as beneficial, especially due to gaining a shared language, through which participants obtain inter- and intra-personal insights, facilitating change of maladaptive behaviours. The internet-based format does not hinder therapeutic alliance and the written space allows for reflection and lessens pressure of the participant to relate to the therapist. The therapist was regarded as indispensable, and the internet-based format provided several benefits.