Music therapy (MT) is a scientific-artistic form of therapy and a therapeutic approach particularly well accepted by adolescents and young adults in the mental health sector. However, there is limited research on how young people perceive their therapy and how they reflect on their music therapy processes. Hence, in “My Tune“, an MT assistive tool for young adults was developed, (pre-) tested, and reviewed in a participatory process. The “My Tune“ tool enables service users to deepen and expand the reflection on their music therapy processes and provides structured proposals for accompanying the reflection processes for the music therapists. In this way, it is designed to help service users to become aware of their experiences and progress and to identify favorable conditions as well as challenges in their MT. In sum, the innovative aspects of this tool are that (1) it was co-developed by adolescent music therapy service users, a caregiver, and music therapists, that (2) service users actively co-determine and decide upon their reflection process(es) (what, when, how, with whom), that (3) young service users are empowered by increasing their agency in MT, and that (4) the “My Tune“ tool thereby enriches MT practice. Additionally, MT research benefited from the insights of developing, testing, and applying this MT tool in a participatory setting.