Alexandria, are you taking enough "breaks"? I would literally get up at least every 45 minutes to an hour and walk around the block. It's been shown that being outside can increase concentration (fresh air, exercise, etc.). Everytime you feel loss of focus, I would get up and make yourself go do something else; it will build up your desire to focus (so you don't have to get up and do something else :-) Short spurts of studying can be more effective than long periods too, gives your brain a chance to incorporate what you have just learned/reviewed.