You do not breathe more CO2 while wearing a mask. Some healthcare professionals wear masks all day without adverse health effects.
On a video symposium from "Doctors for Covid Ethics", a group that spreads and launders medical misinformation, several doctors make numerous inaccurate or inflammatory claims. One of the main claims is that wearing face masks can be harmful to your health, because they can increase the CO2 that you breathe. While breathing too much CO2 is harmful to your health, breathing while wearing a cloth or surgical mask does not affect your CO2 levels. CO2 molecules are much smaller than the microscopic holes in both cloth and surgical masks, and therefore can pass in and out of these masks without problems.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19, including symptoms, prevention, and available treatment, please refer to the World Health Organization or your national healthcare authority.