Trust me, everyone who is revising and 'working-hard' lacks the motivation and energy to do it too. You just have to push yourself through because at the end of the day you can revise and get great/better grades, or you can do little-to-no revision and get mediocre/potentially fail...the choice is yours.
You also don't have to do 8 hours of revision - quality over quantity!
You could sit and read a textbook for 8 hours, or you could write out notes for 15 minutes and I know for a fact which one would be more effective... if you're limited to time just focus on working hard on the topics you struggle with most - aka, prioritise your revision.
IT IS NEVER TOO LATE TO START.