If you want those grades revise. If your finding it difficult to motivate yourself then convince yourself your not doing much at all. Start by seeing if you have any TV programmes/films vagely related and start with a topic you find easy. Then do one you find difficult. This is what I did, and while I never quite believed it the pretence of doing revision meant I actually did some.
Oh, also once you have say done topic x have something to treat yourself with - going out with friends, chocolate, a long bath, whatever.