In my experience, if you're not motivated then nothing except fear or a sudden wake up call will do it. So you have to assume you're not going to get motivated (sorry to be morbid). Make yourself a RIGID timetable: make lists of all the stuff you REALLY need to learn - decide if it's better to make sure the stuff you already know you know well, or to try and get a basic overview of all the stuff you don't already know. Then STICK to it. If you can't get motivated then you have to just force yourself to do it!!
To motivate yourself, I would think of some bad things if I don't get the grades. Some examples of bad things would be not getting into college or university.
i had to hide my network card and change the password on mycomouter to something along the lines of apoirejgiajerglij just so i would do revision. If you can't get motivated, remove the temptation do other things, e.g. go on the internet, check your phone to see if anyone has texted you (this could just be me..)