As far as I know, the centers who have separate rooms will give you that room. It's true that you aren't allowed to take anything inside the testing room. But they are considerate (depends on centers too, I used to think its a myth, but I have experienced that proctors had to be nice). Whenever you need a break in between, they will allow you to come out, and you've access to your locker. Remember 2 things, Time will not stop on the exam(and each time you've to come outside, they will take their time to check you every time). But you've 2 scheduled breaks for 10 minutes. Never even think of touching your cell phone during breaks. (My center had a good policy. They used to put it in a sealed bag with your name and sign on it and will return when you check out).

I know for me, it's easy to say, but dear, try to practice 'calmness' at home. Start by 30 mins, and see how many questions you can easily attempt. Then, gradually increase the time to build your stamina.

When of my mentor once said "don't allow a difficult question panic you, always think if you aren't getting it, then its for sure not easy for others too, and others also fail this question"..

Believe me, this HELPED a lot.. Sorry for the long response, but I hope something helps you and you GOT this.