That sounds similar to my sophomore year. I was taking two AP classes and precalc and chemistry. That year absolutely sucked - it was probably the worst year of my academic history. It is always hard when you first start taking really advanced classes, and that's simply because you are expected to do so much more than you have ever done before (as far as effort, studying, homework, etc.). It is definitely not an easy adjustment. The first semester of my sophomore year was like a living hell for me. Motivation is always the biggest problem. It's always very hard to find motivation to do homework and get good grades because there is not any direct reward from doing so - it's not like a job...you don't get paid or anything.
What you need to do is remind yourself of your long term goals. Do you want to go to college? Do you want to live a stable life and have a job that you enjoy? What do you need to do to get there?
Well, to get a decent-paying, enjoyable job, you most likely will have to go to college. To get into a good college, you need to do well in high school. That means you need to get good grades, do well on college entrance exams, participate in extracurricular activities, volunteer, etc.
And what happens if you don't get good grades or do well on college entrance exams or participate in extracurriculars?
You probably won't get into a good college. Then you'll probably have trouble finding a good-paying, respectable job.
That's how you have to look at it.
Another thing you could do is reward yourself every time you get a homework assignment done (browse LW for 10 minutes, watch TV for a little bit, etc.). Give yourself a treat for every time you achieve something. Just don't get carried away.
You could be the smartest person in the world, but it won't matter if you sit around on your ass all day.
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You're simply the best!
Better than alllllll the rest!
Better than anyoneeeeee!!
Anyoneeee I've ever met.