This randomized controlled trial (RCT) with 250 patients suffering from type 2 diabetes mellitus (T2DM) aims to investigate the effectiveness of the self-guided digital therapeutic covivio. Inclusion criteria are: male, female or non-binary; age ≥ 18 years; confirmed diagnosis of T2DM; elevated levels of diabetes-specific emotional distress (Problem Areas in Diabetes \[PAID\]-20 score ≥ 33); consent to participate; sufficient German language skills. Exclusion criteria are: a diagnosis of type 1 diabetes mellitus (T1DM); current use of continuous glucose monitoring (CGM) as part of one's diabetes treatment; changes or planned changes in medication within 4 weeks before the baseline visit or in the subsequent 6 months; a bariatric operation within 4 weeks before the baseline visit or such an operation planned in the subsequent 6 months; changes or planned changes in psychotherapeutic treatment within 4 weeks before the baseline visit or in the subsequent 6 months. In addition, participants will be excluded from the study if they did not wear the CGM sensor for at least 6 of 7 days at baseline (before randomization), providing a minimum of 96 hours of glucose values including at least 24 hours overnight. Patients will be randomized and allocated in a 1:1 ratio to either an intervention group, in which they will receive access to covivio in addition to treatment as usual (TAU, n = 125), or to a control group, in which they will receive only TAU (n = 125). Co-primary endpoints of this trial will be the between-groups difference in the PAID-20 total score in the intervention and control group at six months, adjusted for the PAID-20 baseline score as well as the glucose management indicator (estimated HbA1c) assessed using continuous glucose monitoring devices (for a period of 7 days) at six months, adjusted for the baseline score. Secondary endpoints will be diabetes self-management skills, body mass index (BMI), and quality of life.