Try and plan things out: e.g. when you are going to work on what.
I dont suggest napping for 10 mins but at night try to make sure you are not distraccted or stressed and get a good sleep; itll make a world of difference.
As Im only in year 11 finishing GCSE I cant relate to your exact situation, but I do know the feeling of not having the energy to work, and its not a good one.