Sounds like you don't really have a proper way to study. A lot of people struggle with this out of a classroom environment and don't know how to self study effectively.
Just reading the book isn't going to make anything sink in, try making your own notes. Also, you may have over done it, if you try to learn the whole thing in one go or one evening it will never sink in, you need to come up with a timetable to structure your learning and break it down into chunks. Then you will find it sinks in more over time. Also taking regular breaks, I could never go much more than 45 mins of study before I wanted to take a break, and that worked for me getting through the ATPL exams.
Air Law is clearly one of the driest subjects, just awful for many people. I would recommend the PPL perfector as a Q&A book, I've seen many students do very well in all the subjects using this.