The answer is simple. You need to do a lot of practice. This is one of the points that test-takers need to overcome - the monotony of dealing with many questions at one go and the problem of remaining focused during this relatively long time.
You need to make sure that you are not tired before doing a test. Also you could try to deal with a smaller number of questions at first and working on your concentration span. Observe yourself. How long do you stay concentrated? How many questions can you solve efficiently? Try adding one or two questions per day and after 2 or 3 weeks you will see a considerable improvement.