On motivation, focus and mental state I have a bit of a meta-question, but first let me give you some background. I used to be a very inquisitive kid, interested a many disciplines, in particular mathematics, computer science and physics. I have never been hard-working, or never had great focus, but I had a very good intuition and ability to learn, which propelled me through high-school and beyond with great success. I got into all universities I applied to, everything was great. Towards the end of high-school, however, I started struggling with depression. I did not worry too much about it at first (its main impact was on my sleep, leading to insomnia, etc). 
During my years at university, however, it became rampant. Today I am in my third year undergraduate. My grades so far have been decent (averaging above 75%) but I feel I am not where I should be. It is very hard to explain in words, so I hope one of the person reading this will have personally experienced it, but I feel in a "braindead" state. I lack motivation to study my courses; I lack focus; I get nearly obsessed over simple concepts if I don't fully understand where they come from.
I would greatly appreciate if anyone could share a similar experience, or any advice as how to deal with it. I doubt reading a single paper will get me back on track, but maybe there are mathematical wonders out there, or enlightening blog posts which could guide me. I am sure many people have gone through similar phases.
Thanks!
 A: This question will almost certainly get closed as "off topic" or something, so I have to type fast.
Maybe you'll get some good ideas, here, but I hope you are looking for help elsewhere, too. Nobody here knows you, and our only understanding of your situation comes from the few words you have written. Random people on the internet (like me) are useful, sometimes, but they are no substitute for counselors, teachers, friends, and family.
So, talk to your friends and family. If there is a teacher you're close to, talk to him or her. Talk to a counselor if you can find one. Perhaps you ought to seek medical help. How was your depression treated when you were in high school? 
Inspiring pieces of mathematics are probably not the solution to your problem. 
I experience depression and the "brain dead" sensation from time to time.  For me, it's often a result of tiredness, frustration, or temporarily damaged self-esteem. My solution is to engage in some aimless simple-minded pleasure for a while. Read a book (not a math book), go for a walk or a run, watch a stupid movie, whittle a piece of wood, kiss my wife, sleep in the sun, stare at the ocean like a zombie, play with the cat, etc, etc. Getting close to nature seems to help me. Eventually, the energy and the will to do something more meaningful come back. But that's my approach; I have no idea whether it will work for you. Anyway, you're not alone. Good luck.
