Make a plan. Try to fit in the chapters each. It'll make you realize that there is enough time (and hope) to complete that revision before your exams.
Personally, my plans work better when am working on my own-- all alone. I avoid school, skip tuitions and try to avoid all contact with the outside world. It's just 20 days. Your social life will not spike down if you do.
Also, cut down on the sleep. 5 hours a day is just sufficient with few 20-30 minutes naps when you feel tired. Your time is very limited, so do value it.
Be strong and don't allow yourself to feel demotivated-- you can't afford it, there is no time!
You might talk about it with someone if you want but I'm skeptical of it. You might get into long unnecessary discussions which aren't really gonna help you out. After all, the exams, you will have to give them yourself. Might as well start getting used to being your biggest support.
Wake up early morning. Write down what you have to do. Close your eyes and remember why it's important to you.