Lose my nerve while doing research
I think that you may experience what can be characterized as Frustration.


  frustration is a common emotional response to opposition, related to
  anger, annoyance and disappointment.


As you're reading scientific papers, you're on higher level of scientific work than is expected from graduate student - I did not read too much papers during my MSc.
And it depends also what is expected of you by your colleagues, so compare whether your frustration is not combined with Imposter syndrome mentioned in comments.

E.g. Authors usually spent several months or years to produce some results and to write a paper. So understanding the paper may take also some time to readers. Other problem is that some papers are so brief thus it's rather a problem of the authors if they can't explain their scientific problem or solution to somebody outside their domain niche.

I have been experiencing frustration personally many times during my MSc, PhD and my current postdocs projects. When it happened I gave myself a time limit. So during that time limit it's good to consult the problem with somebody else. If I don't make any progress and frustration continues after the time limit, I give up and move to somewhere else. 

The limit and tolerance to frustration is very subjective. Nobody should expect you to grasp all problems and in short time. My oppinion is that mental health is more important than scientific achievements.