Good for you for recognizing that it takes hard work and dedication to get good grades. Some students fall into the trap of putting all their time and energy into having fun and none at all into studying. It sounds like you're tipped the scale in the opposite direction, which can also cause problems.
The trick is to figure out how to keep an even balance between school work, extracurricular activities, relationships, and just plain fun. When that balance gets out of whack, every area in your life is probably going to suffer. To restore balance, schedule some fun activities just like you would schedule a sports practice, family event, or final exam cram session. Literally get out your calendar and write down two or three fun things you want to do each month. Could be a movie that you're looking forward to, a game you want to attend, a trip to the mall, or a favorite TV show. As long as you make sure that your social life doesn't interfere with important assignments or tests, you'll find that having something fun to look forward to can really help you stay focused during school and study sessions.
In addition, it's a healthy idea to set aside at least an hour or so each day just for relaxing. Decide in advance when you want to take your study break. If you find it hard to take a break, ask a parent or friend to remind you to close the textbook at a certain time. When the time comes, put down your pencil and set a timer or an alarm clock for one hour. When time is up, go back to the books and reward yourself with additional time off if you finish studying early.
Even though you may be spending a little less time studying overall, you'll find that your time is more productive if you remember to have a little fun, relax, brush your teeth, shower . . .