always worried about not breathing

i’m always worried that my brain could not be getting enough oxygen. it’s not only when i’m sleeping but is it possible when i’m sitting down. 

in the case of sitting down is this possible bc the diaphragm is restricted and what if you aren’t able to tell that you can’t breathe, what would be the signs that you can’t? and would happen if you can’t in that case? do you pass out and start breathing again or what? 

and in the case of sleeping, sometimes i awake feeling like i wasn’t getting enough oxygen and out of breathing some instances. what would happen if you aren’t able to breathe at night? would you wake up ? or is brain damage possible without knowing 

also what about strangulation, for example from a tight jacket or jumper around the neck. 

also what about carotid artery dissection? 

overall i’m worried about brain damage which could occur from the lack of oxygen