CPR isn’t like in the movies where it “revives” people from unconsciousness. Your job is to manually pump the heart by breaking the cartilage that holds the ribs to the sternum and using the sternum like a pressure plate to force blood through the heart. The oxygenated blood will then continue to circulate feeding the brain and helping to prevent damage while emergency crews with defibrillators and actual medical equipment can be brought in to place them on life support.
It’s critical to understand the purpose, you’re not going to be bringing anyone back with CPR, but you just might keep them from coming back a vegetable if you do the compressions correctly until help arrives.
Even if you’re not able to access their airway or provide breaths, merely the act of correct compressions continuously applied is usually enough to keep the unconscious brain supplied with oxygen. Provided help arrives within about ten minutes that is. This is why the “ABC’s” of CPR have been changed to CAB, after the realization that chest compressions are the single most important factor in CPR.
If you can give breaths you should, but keeping the blood flowing is your upmost priority.