I get the feeling that you are trying to learn all this stuff from text books alone, if this is the case, it can be very difficult to retain. When I took my FE written many years ago (before I had any experience on Jets), it really helped to have visual aids where you could see how these systems worked and interrelate with each other.
If you, like me at that time, do not have the practical background, I suggest you take some classes which can help you with understanding this stuff, alternatively, find an engineer ( that's the member of the Cockpit crew that sits sideways most of the time and blocks the new Flight attendant from getting to the Captain) and have him/her help you over the hump on this stuff.