You don't need motivation, you need action/discipline - you just need to do something.
Don't think about all of the subjects you have to revise for right now; just pick one, then pick something from that subject to revise and just do something about it - read over old notes, google if you're confused, go through a revision/textbook and so on.
Worrying won't get you anywhere (see quote in my sig.); you just need to start doing something regardless of how unmotivated you feel.