It comes down to controlling your own thoughts.
Other stuff like deep breaths, diet, sleep is secondary as its in the exam where the destructive thought processes start.
The key thing is, to recognise that you entirely control your own thought processes, although it is not easy and it requires practice. People think they are out of control and its like some outside force that imposes those thoughts on you but you run your own brain.
Boxers or martial artists have to mentally train themselves to stop negative thought patterns developing as they can't afford to allow fear to sneak in while they are fighting and it's the same in an exam. Remember the phrase "stop listening to yourself, start talking to yourself".
Read stuff about sports psychology, mental toughness and it talks about this kind of thing. But you also need to start practicing it from now. Practice stopping negative thought processes as soon as they develop and replacing them with positive thought processes. You get a lot of opportunities at stuff like revision time as sometimes thoughts start to creep in like what if I don't do that well blah blah....leading to my life will basically be over. Just stop those thought patterns as they can never help you, they can only harm you.
Emotions always follow thoughts, not the other way round. If you control your thoughts you can manage your emotions.