Ive experienced similar symptoms of tiredness and demotivation, I used to think there was something wrong with me like a physiological condition. I was at my worst during my third year of University. Im not completely cured but here is how I got over it:

1. Went travelling after I graduated; exciting; helped me mature and become mentally stronger - made alot of friends.
2. Eat well; stay hydrated; go to the gym - how can my body feel depressed and down when physically I feel so healthy? (in my opinion) My gym membership has really helped me through hard times, helping me through those dark and cold days and days of revision.
3. Keep busy; learning a new language; social groups - make more friends.
4. Try to enjoy your academic work; short bursts over a longer period of time rather than stressing over a few days before deadline.
5. Do something new e.g. I moved to London to do my Masters degree and now I feel energised because life is so hectic and action packed. Asoon as I return home I begin to feel down and bored.
- basically its all about routine - having a goal/aim - setting targets - making progress and being proactive***

^ this is just my experience so it may no be of use... meh.

peace
x