I'll try my darnedest in the forthcoming semester to have better sleeping/eating/exercising habits. Thank you everyone for taking the time to reply. (I was thinking to add a few more details to my questions and replies that I think are unique in my case, i.e., I hail from a third-world country, the first time I attended school was in America at the age of 12, and I have severe social anxiety, which, very strangely, I trace back to having conflicts in living in a multi-cultural family - but I decided not to add these details, since it would make for a disheveled, disorderly discussion). Thanks again everyone. :)
Also, this made me chuckle a little bit, just because, coincidentally, this is on the front page: http://news.ycombinator.com/item?id=660720.
I wouldn't even consider nootropics until you're already getting plenty of sleep, eating well, exercising regularly, and not using too much caffeine or alcohol. Those things have a much bigger impact on your state of mind. Nootropics are attractive in part because they represent the dream of sharpening your focus without all that boring lifestyle stuff.
Piracetam is cheap, studied (at least compared to the rest...), and doesn't have any particularly bad side effects. If you're going to take anything, that might be a better place to start than amphetamines (!) such as adderall. Of course, I'm not a doctor, but I doubt you are, either.
I've had the best results with green tea (which includes l-theanine) and getting enough sleep, FWIW. (Also, meditation helps.)