Integration of different psychotherapeutic approaches and interventions has become a crucial aspect in psychotherapy practice. Although different conceptual models exist, the experiences of integrative therapists are often ignored. The aim of the present study is to reach a better understanding of therapists’ experiences and challenges while integrating new therapy elements. Twenty-six relatively inexperienced therapists took part in the interview study, which was embedded in a randomized control trial. The goal was to carry out 3 psychotherapies while integrating either components of Emotion-Focused Therapy (EFT) or Self-Regulation (SR) into their treatment as usual. Therapists were interviewed during 3 different time points throughout their therapies. In total, 97 interviews were transcribed and analyzed using MAXQDA 12.3 following Mayring’s (2000) qualitative content analysis. Results indicate that therapists implicitly followed heuristics to assimilate new therapy elements into their originally trained approach. With the integration of therapy elements from conceptually more foreign therapy orientations (i.e., EFT), therapists seemed to experience a slightly higher challenge, but at the same time, perceived it as a chance to enrich their practice with the result of a more individualized, responsive practice. 62% of all therapists reported a thorough change in their therapeutic style. The results suggest that assimilative integration is a complex, multidimensional process guided by various principles regarding the fit of the new therapy elements to patients and therapist characteristics in a responsive manner considering the therapeutic alliance. The findings stress the importance of therapists’ relational flexibility.