Past Papers is key, as you get a feel for the exams- you'll then realise that it isn't that daunting after all. Also, when it comes to revising, get the specification, and tick off everything you know, whilst making notes on each topic. The topics that you haven't yet covered, or completely understand, use revision websites such as 'mygcsescience'. There, you are able to go over videos covering the topics that you don't understand (but be sure to go over the topics in the specification for your exam board).
When it comes to motivation to revise, I would suggest visualising the kind of grades you want to happen (an A*). You then have to work hard to ACHIEVE that goal; the though of failing should be enough to motivate you also.
Best of luck in your examinations- hope this helps.