Take a break when your mind needs it. Don't study all at once. Do it in bits and pieces. Study a little, then do something relaxing and/or fun, then study some more, and keep that cycle going. If you cannot figure something out immediately, put it down and come back to it, when your mind has had time to freshen itself up. Don't get angry with yourself when you can't get something right the first few times. Don't do anything if you are too tired to do it. Rest, take naps, eat snacks, and that way your brain will be in full force next time you pick up the book. You study best when your brain is rested.