I am trying to improve my problem solving skills by solving olympiad problems (Putnam, IMO, etc). So far, I have discovered that problem solving is somewhat like panning for gold: you think of all the different angles from which to approach the problem, and try them one after another until you find a solution.
Unfortunately, I find myself losing concentration while actually trying to solve the problem. I do not lose concentration doing simpler mental tasks, but handling numbers and models and expressions in my head is tiring. I lose my train of thought and get distracted frequently, which slows me down and forces me to start over.
This does not happen when studying simpler subjects; in fact, I only realized that I have this problem when I began practicing olympiad problems. Is this a common experience for most people when they first begin? If I want to improve my concentration, should I keep practicing these problems, or should I do something different?