My personal mantra is to not worry about it. Now there's a difference between not worrying and not caring (which is the difference between being "smooth" and being a hippie).
You still care. Work your ass off and study, and get done what you need to get done. But don't study because you "have to study", study because you're interested in the material and want to learn it. Don't worry about "having" to study for this class; you're reading the material because it's intriguing.
When you take a test, don't worry about it. Just do your best and be happy, because if you fail it, well there's no need to worry about that because you've already failed it for whatever reason.
Don't stress out about not getting the perfect grade. It isn't the end of the world, not even remotely close to making a difference in your life, regardless of how important it may seem to you. Don't let things get you worked up, just do your thing and be happy with it.
I'm a personal believer that attitude is the primary determinant of your success or failure in any endeavor you undertake. My freshman and sophomore years I didn't care, and I made straight B's with a few sprinkled A's, in pretty easy classes. This year I care, but I don't worry. I just do my thing and I put in quite a bit of effort, and I take what I get. I'm taking 3 AP science courses and an AP history course in a 4 block schedule and I'm making all A's. (Biology, Chem, Physics, US History)
I see other students in my classes fretting about little things that don't really matter and they struggle with just a single honors course. Maybe I'm just some kind of strange outlier, but I think being too neurotic/trying too hard will doom you just as much as not caring at all. Try to find a happy balance and let it come to you.
Also, remember that the learning curve is just that: a curve. The learning curve is not a linear Time vs Amount Retained graph; it's called a curve for a reason. The return you get from an increasing amount of studying is less and less. A lot of students think that they can take a super hard schedule and just take on an extra 10 hours of studying each night and cover it. It doesn't work that way; it's an exponential curve, not a linear.