I'm a reasonbly bright student, and I have set my sights high for the future. How do I make sure I study?

[Wandering Limbs]

You seriously need to calm down or things will just get worse.

If you're topping the classes then obviously you're doing well, I think you need to put less pressure on yourself and work on creating a balance. I know to someone who feels like having a social life is the enemy, saying that you should go out more sounds nuts, but you just need to take your mind off studying for a while. I'm not saying go party every night. Just set aside set times for study and socialising, but make sure that the times are flexible, because they won't stay exact.

Break up your study times with something you enjoy in-between, but make sure you don't use it to procrastinate, set a timer or something. This will keep you on task and focussed when you sit back down to start again.

If you're finding you're overly fatigued without having overly exerted yourself, there's a chance it could be something health related. Something such as being anaemic, or diet or lack of exercise can drastically affect your levels of alertness, so maybe look into checking that out.
I went through all of year 12 last year thinking that I was overreacting to how tired I was (I found it physically and mentally impossible to study because of it). Turns out it was related to a pre-existing medical condition that I already knew about. Despite doing no study, I was lucky enough to still get a good mark and get into the university that I wanted to.
Make sure you don't let it get out of control though, because while I admit I'd already had depression on and off since year 10, it got REALLY bad last year because my marks were dropping, but there was nothing I could do, and that made me more anxious and stressed and it was just a vicious cycle.

I probably haven't actually helped much I'm sorry, but I really do wish you good luck. I'm starting my 3 year undergraduate course before applying for medicine this year, so I've got 3 years ahead of me where I need to get the highest marks possible in order to get into med - so I suppose we're sort of in the same situation.
Good luck!