One of the most difficult part of doing an exam it the time constrain.
The best way to study for an exam is to time yourself and to try to do problems in less than 10 mins each without using
the book or looking at solutions until you are done.
When writing an exam I then take the exam and time myself.
I do the exam carefully with everything written nice and with every detail.
Then I make sure that it takes me one third of the time to that it should take the students to write it up.
For example a 50 min exam should take me from 16-17 mins to do.
When doing practice exams or problems, if you still feel like there wasn't enough time,
perhaps you want to reevaluate your studying and test taking strategies.
I am more than glad to help you with tips on that and to have a 1-1 talk with you about it.
One other reason for felling like you don't have enough time could be that you are approaching the
problems using a method which is not the most efficient. Always try to think if there is a better shorter way, another
technique that we learned in class that can answer the question in a more efficient way.
Here is a link with tips on taking timed exams.
Here is another link with exams taking strategies in general.