Use your diaphragm to breathe instead of your chest. Think of breathing through your back while expanding your ribs.
When you start wearing a mask during your workouts, you may need to reduce the intensity of sessions, especially cardio. You can also increase rest periods while strength training. Continue to focus on breathing through your nose and aim for your exhales to be as least as long as your inhales, as that is how CO2 is expelled. Inadequate exhalation of CO2 is the main reason it may accumulate in the bloodstream when wearing a mask.
Our thoughts affect not only our minds, but our bodies, too. Consciously and regularly checking in on how you feel can help you physically and mentally relax during activity.
Masks lose their effectiveness when they are wet. If you plan to workout for more than 30 minutes, bring a spare mask along just in case. Also, wash your masks regularly.