Inclusion Criteria:  People with female-assigned reproductive systems aged 18 to 45 years Medically confirmed endometriosis diagnosis Impairment of quality of life due to endometriosis defined as a value of ≥ 15 points across all scales in the Endometriosis Health Profile 30 + 23 (EHP-30 + 23, (Jones, Kennedy, Barnard, Wong & Jenkinson, 2001a) Adequate German language skills Access to a personal computer (PC) / laptop and adequate internet connection  Exclusion Criteria:  Serious mental disorder: unipolar severe depression (Beck Depression Inventory II: raw value ≥ 29), presuicidal syndrome, bipolar affective disorder, unipolar mania, psychotic disorder, alcohol or substance dependence Psychotherapeutic treatment: currently or within the last two years Use of benzodiazepines (prevents learning processes that are important for psychotherapy) Changes in the intake or use (e.g. start / change of dose / discontinuation) of antidepressants and hormonal contraceptives within the last three months Current or planned intrauterine insemination (IUI), in-vitro-fertilisation (IVF) or intracytoplasmic sperm injection (ICSI) treatment with hormonal stimulation within the next eight months Suffering from one of the following diseases: malignant tumor, in particular breast, cervical or ovarian cancer, ulcerative colitis, Crohn's disease, a known bacterial or viral infection (e.g. tuberculosis (TBC), hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV) Current pregnancy or birth of a child within the last six months, breastfeeding within the last six months