Some situations in life can be very stressful but you should know that it's not the end of the world if you don't meet up to its criteria or demand. I'm not saying take your exams lightly and fail, rather try your best and leave the rest in Allah's hands.
When you know things are out of your control and you can't do much then just do what's necessary and try your best.
I feel you've stressed yourself out way too much. This can only make things worse because it won't help you achieve your goal at all.
I would suggest you do the following:
1. listen to the Holy Quran. This act is proven to put one at tranquility.
2. Plan out your work. Sometimes the amount of studying you need to do seem overwhelming because you dont have the framework. Plan out a study plan. For each subject write down the topics and sub titles that need to be gone through and then continue that way. This will half solve your load.
3. Take some time off all the stress and do something you enjoy most like swimming, yoga, etc etc anything that will physically involve you. This will pump up your heart, get your body rushy and revive your strengths. Deep breaths are ideal to redirect your thoughts and concentration. This act pumps up extra oxygen to your brain and helps you feel ' in control'
4. Make dua. Speak plenty to Allah (azj) and tell him of your worries and concerns. When you vent out your worries and tell it to the being who has EVERYTHING under HIS control, you will feel energetic and hopeful.
5. Recite the Dhikr "la hawla wala Quwata illa billahi al-Aliyil Adhim"
6. Make sure you're eating healthy (take your vitamins religiously) and also makesure you're getting adequate sleep.
Good luck. I'm sure you will do wonders in your exam.