I've struggled with similar lack of motivation for a while now, the things that work for me are setting out a very strict timetable for each day, with work blocks clearly marked for a few hours here and there, and forcing myself to follow it exactly, and also withholding certain rewards, in my case chocolate, and only allowing it after a certain amount of work.
that or post all your revision notes around the room where you just happen to see them and revise almost by accident