Remember the reason for which you decided to attempt CSS. Remembering my initial reason for CSS always helped me get back on track.

However, there are times when you don't feel particularly demotivated but you have had enough of history or current affairs or .... you name it

To deal with such situations, you need to find an interest which will be productive, helpful and allow you to have a break at the same time. It could be watching some documentary on a culture or religion or about some social issue. It could be a documentary related to any of your optional subjects (you just need to search the internet properly and you can find virtually anything). Or it could be a book on something that interests you (science and philosophy books helped me at such times).

If nothing works, try exercise (believe it or not, it works wonders).

For some people interested in fiction, some classic or even modern novel might do the trick.

And remember, if you have time on your hands (more than 3 months), and you don't feel like studying for a while, try not to force yourself to study. At that time, do what your mind desires you to do
Once you satisfy that distracting desire, you wil be able to concentrate better on your studies and make up for lost time quickly.