The goal of this clinical trial is to test the efficacy of Acceptance and Commitment Therapy (ACT) as a psychotherapeutic treatment for patients with Bipolar Disorder (BD). The following questions will be investigated: * The efficacy of ACT in reducing depressive symptoms in patients with BD. * The efficacy of ACT in improving the quality of life in patients with BD. * The efficacy of ACT in improving functionality in patients with BD. * The efficacy of ACT in improving sleep quality in patients with BD. * The efficacy of ACT in improving psychological flexibility in patients with BD Participants in the clinical group will undergo 12, 2 hours, weekly sessions of Acceptance and Commitment Therapy, in addition to their regular pharmacological treatment. Participants in the control group will undergo 3, 2 hours, weekly sessions of Bipolar Disorder Psychoeducation Therapy, in addition to their regular pharmacological treatment. Scales and assessments will be used to measure study outcomes on 5 different time-points: pre-intervention (month 0), post-intervention (month 3), follow-up 1 (month 6), follow-up 2 (month 9), follow-up 3 (month 12). Mood scales will be assessed every 2 weeks for the duration of treatment and every 4 weeks during the additional 12 week follow-up period. Results will be compared among both groups to assess the effectiveness of Acceptance and Commitment Therapy as an intervention for syndromic and sub-syndromic depression in bipolar disorder.