Losing focus in an exam, how does it affect my goal - grad school
I would try to find some of your class mates, that are performing very well on the tests and ask them for some advice. Regarding what sources they are specifically studying, how they decide what material to prep for the exam and what not to etc.

If you don't know anyone yet that is performing very well, this is a great opportunity to make some new friends. Taking tests is really an art form all of its own, and I'm sure those in your class that score highly will have some insights they could share with you about how to sit and prepare for a test well. 

Also ask your faculty if they have old papers, you could practice with, you might also ask a member of the teaching staff to perhaps do a mock invigilation for you, with an old paper, and give you feedback about how you did, and where it fell short. Practice makes perfect. 

Also isolate the reason you are not doing well in the tests. Is it because you simply do not know the answers? Or are you running out of time? Or are you not reading the question properly under exam conditions? Once you know this you will be in a better position to develop a strategy to improve in the key areas that are impacting your scores.