This study is a randomized, controlled, prospective trial with a 6-month follow- up. A newly developed psychoeducational treatment and education programme for diabetic patients on an insulin therapy who use flash glucose monitoring (FGM) will be tested compared to a waiting group. Primary outcome variable is the difference in glycemic control between baseline and the 6-month follow-up. Secondary outcome variables are: time-in-range, frequency and duration of hypo- and hyperglycemic episodes, diabetes-related distress, depressive symptoms, health-related quality of life, diabetes self-efficacy, self-care behavior, and hypoglycemia awareness.