In order to avoid suboptimal psychotherapy, research needs to highlight and analyze obstacles in such treatments. This clinically oriented article brings together empirical material of unsuccessful psychotherapy with young adults; empirical material on the therapists’ views of the same therapies; and theoretical perspectives on mentalization, therapeutic alliance, and young adulthood. Through a secondary qualitative analysis, it presents a tentative process model of how suboptimal psychotherapy with young adults develops, how it could be handled clinically, and possibly prevented. In three studies, experiences of young adult patients (aged 18–25; n = 27), in psychoanalytic therapy at an outpatient clinic, who did not improve from therapy (defined as no reliable and clinically significant symptom reduction) and/or were dissatisfied, and their therapists, were analyzed. Patients described experiences of not being understood and not understanding therapy, whereas therapists described patient non-commitment. These results were compared from the developmental perspective of mentalization in young adulthood. The primary grounded theory analyses and secondary analysis resulted in a tentative process model of the development of suboptimal psychotherapy with young adults. Suboptimal therapy is described as a vicious circle of therapist underestimation of patient problems, therapeutic interventions on an inadequate level, and diverging agendas between therapist and patient in terms of therapeutic alliance, resulting in pseudo-mentalizing and no development towards agency. A benign circle of successful therapy is characterized by correct estimation of patient problems, meta-communication, and the repair of alliance ruptures. One clinical implication is that therapists of young adult patients need to establish verbal and nonverbal meta-communication on therapy progress and therapeutic alliance. The importance of the patients’ present mentalization capacity and adjusted interventions are demonstrated in an example. Research in the field should be process-oriented and investigate the effect of meta-communication and interventions targeted to foster therapeutic alliance based on this theoretical model, particularly for young adults.