Many psychiatric disorders and symptoms have been associated with impaired metabolic control in type 2 diabetes; several studies focused on non-pathological psychological features. Aims of this observational, longitudinal study are: the assessment of the impact of a wide range of psychological factors on metabolic control in type 2 diabetes; and the development and validation of a simple questionnaire to assess the impact of psychological factors on therapeutic success. To identify psychological factors interfering with attainment of glycemic targets, a prospective 1-year study was performed on a sample of 250 patients with type 2 diabetes. The impact of identified factors on therapeutic outcome was then subsequently verified on a further, independent sample of 200 patients. The first phase of the study allowed the development of a 19-items questionnaire, the Psychological Predictors of Therapeutic success in Diabetes (PPTD) questionnaire. Validation analyses showed that the questionnaire was able to predict therapeutic success. Patients with HbA1c ≤7% (53 mmol/mol) at follow-up showed higher test scores than those with HbA1c >7% [31.0 (26.2; 35.0) vs 28.0 (23.0; 32.0); p = 0.016]. The attainment and maintenance of therapeutic goals in patients with type 2 diabetes depend on a wide range of factors. The PPTD is an attempt at condensing the complexity of psychological factors affecting glycemic control in a simple and easy-to-use self-reported questionnaire, which can be used in wide-scale research.