Ultra-brief Psychological Treatments for Emotional Symptoms and Disorders

The main objective of this randomized clinical trial is to study if various modalities of brief (eight sessions) and ultra-brief (four sessions) transdiagnostic cognitive-behavioral treatment work for decrease emotional symptoms and disorders, as well as to find out which variables of cognition and of the intervention process itself are involved in the therapeutic improvement. The investigators will compare the interventions of four groups: brief group treatment (1), ultra-brief group treatment (2), ultra-brief individual treatment (3) with the ultra-brief relaxation (control) group. The main questions the study aims to answer are: * Will the brief and ultra-brief treatment formats works better than the relaxation-based control group? * Will results obtained between the brief group therapy group and the ultra-brief group therapy group be similar? * Will the individual ultra brief therapy be more effective than the two group therapies because of common factors, such as the therapeutic alliance? * Will group therapies be more beneficial for cost and in terms of capability to reduce emocional symptoms than individual therapy? Participants will be randomly assigned to each of the groups and will receive the corresponding treatment, with different number of sessions. They will answer a series of questionnaires at the beginning and at the end of the intervention, as well as 3 months and 6 months after the end of the treatment.