This doctoral thesis addresses heterogeneous psychological suffering within a predominantly psychiatric paradigm, emphasizing the high prevalence of comorbid mental disorders. While much of the published empirical research focuses on trials targeting single disorders, this thesis aims to explore and evaluate novel treatment approaches for populations experiencing diverse psychological suffering. It seeks to bridge the gap between traditional disorder-based treatments and the emerging trend of idiographic designs and processes of change. The first clinical trial focused on treating co-morbid insomnia and social anxiety disorder using sequential cognitive behavioral therapy protocols (CBT-I and CBT-S). Findings suggest potential efficacy in reducing symptoms of both disorders, with notable improvements in insomnia symptoms. However, treatment effects still varied among participants, making it difficult to draw clear conclusions regarding efficacy. The second trial investigated group-based Rumination-Focused Cognitive Behavior Therapy (RFCBT) for individuals with depression, anxiety, and insomnia. Results indicate significant improvements in insomnia symptoms post-treatment and at the 2-month follow-up, with potential effectiveness for depression. However, no significant effects were found for anxiety, worry, or rumination. The third trial explored feasibility and preliminary effects of a process-based psychological treatment informed by Relational Frame Theory for individuals with comorbid mental disorders (Process-Based Behavioral Therapy; PBBT). Interpretative Phenomenological analysis revealed participants' varied experiences, emphasizing challenges in emotional engagement and the therapeutic process. Mixed findings from both the qualitative analyses and the suplementary self-rating scales underscored the complexity of treatment outcomes, highlighting the need of more research on treatment approaches based on Relational Frame Theory. Overall, the thesis contributes to addressing and understanding the complexities of heterogeneous psychological suffering and clinical research. Further research should continue to explore idiographic designs and process-based treatments while carefully defining and tracking processes of change. This is particularly important for individuals who do not benefit from current evidence-based treatment approaches.

This doctoral thesis addresses the pervasive issue of psychological suffering, underscoring the high prevalence of and common comorbidity between mental disorders. Despite the undeniable efficacy of psychological treatments in alleviating suffering, a significant proportion of individuals fail to benefit from evidence-based approaches. Research on treatments for people who meet criteria for several disorders is scarce and current evidence-based treatment approaches still tends to target and or be evaluated on delimited disorders. This raises the question of how to address heterogeneous psychological suffering.

Acknowledging the multitude of angles and paths available for evaluating and developing psychological treatments, the overarching goal of this dissertation is to explore and evaluate novel treatment approaches in populations with heterogeneous psychological suffering. By bridging the gap between nomothetic designs, disorder-based treatments, and the emerging wave of idiographic designs and process-based treatment approaches, this thesis also aims to explore some of the scientific dilemmas that arise from developing and evaluating psychological treatments in a predominantly psychiatric paradigm.