I lighted upon this sterling answer by virtue of user Hepth at http://www.physicsforums.com/showthread.php?t=677222:
I too get mental fatigue if I'm working too hard. Usually my problem is if I work on research (read papers/do math/program mathematica) for 8+ hours it takes another 4+ hours for my brain to slow down and I can relax. This leads to problems as if I work late, maybe to 9pm before going home, and then get in bed at 11pm, there is 0% chance of sleeping for another 3 hours due to my mind just running and running. Its horrible, and then the next day I'm even more fatigued and tired and subsequently get less done.
The key to solving this for myself was to :
1) Go to bed early
If you're a student, this is difficult. But stop studying late into the night. Set a curfew where you don't do homework or study after say 8-9pm. Give yourself time to relax.
If you're of working age, spend time with your kids after work (most important), but don't check your emails often/fret about tomorrow's workload/etc after a certain time. Relax, and do something you can enjoy without a ton of mental stimulation (take the wife/SO to a movie, the mall, the modern art museum, etc.)
2) Get up early
If you went to bed early, you should be able to get 8+ hours of sleep and get up at a decent time. The discipline needed to get out of bed quickly and get ready for the day is tough to acquire, but if you nail this down in college, you'll be ready for those 1am, 2am and 3-5 am wakeup calls from your newborn without feeling like your heart is going to explode every time the "alarm" wails.
3) You MUST take breaks during your workday.
Sometimes I feel like I shouldn't. I'll be on a roll, working hard, making progress, skip lunch, then all of a sudden its 6pm. I FEEL like I was getting a lot done, but I really just wound myself up and zoned-in. While I was working for the full 8-10 hours, I didn't get 10 hours of work done.
Instead, if I take a break every hour (or a little less), and go walk and get some water, get some fresh air, I find that during this break my mind will reorganize the priorities of what I'm actually trying to accomplish, and I'll nail down a single task as soon as I sit back in my chair. This also helps avoid the brain-burning overload of studying/working for long periods of time.
4) Stop browsing the internet when trying to study/work.
Remove facebook/etc from your bookmarks, don't save it so it stays logged in/etc. Make it difficult for yourself to access those sites. While you might think browsing the internet is harmless and basically a "break" from working, its not. You're still thinking about what you're reading, and its a non-stop flow of new, but worthless, input.
As for the whole "sick of learning new ideas" problem, it sounds like you're in the part of your studies where you're working on a bunch of difficult material that you have no interest in. If you were interested in it (like I was in physics) you'll have no problem studying it and learning it quickly. But if you think its worthless for you to learn and just hate it (Organic Chem for me, do I really NEED to know how to properly identify and name 1 cis-3 methylcyclopentane ?) then you just need to "soldier on" and try your hardest to be interested in it.
It's the interest in a subject that makes learning easy; nothing is actually too difficult to learn.
As an aside:
Organic Chemistry was my lowest grade in undergrad (I think like 77%). I was "placed" into it due to my entrance exam in my first semester of college. I was taking 23 credit hours of courses and way overloaded. I struggled to get that 77%, hate the class, and it took up about 80% of my study/homework time as we had to complete homework assignments in the chem computer lab every other night.
The last semester before I graduate I'm told that Organic Chem was NOT required for my degree, that the school should not have made me take it, and the pre-requisite (Principles of Chemistry) IS required, and though I tested ahead of it I didn't get credit, and I'd have to go back and take it. I ended up doing well, but I realized it's errors like this that are the reason so many people drop out in their last semester.