Mental calmness during timed test taking I am preparing for a timed post-graduate test (for example, GRE, GMAT etc.). I get hung-up on questions where I don't immediately find a path, and then beat myself over not doing it fast enough while I am still on the question. This further increases my time on the question by taking focus away from the problem at hand. 

Additionally, the above gets compounded with thoughts of inadequacy- "I am dumb" etc. etc. This adds to the time guilt and takes away from thinking clearly about problem solving.

If you've had a similar experience, what did you do during test taking to overcome it?
I would suggest getting used to self-proctoring practice exams within a certain time limit. Keep at it until you figure out how to determine when it is fruitful to skip a question. Come back to it later when you have an ah-ha moment or finish the exam. As for worrying about spending too much time or self-depreciating yourself, try to minimize it. Remember your successes. Take time to build up your confidence both inside and outside academia and strive to do the best you can do.

Think of every difficulty as a chance to be humbled, and use that outlook to find wisdom in the experience. In grad school, I'm sure you'll encounter this same feeling just by speaking with people whom are your senior.