Psychologically it is quite a normal phenomenon that you are facing during study Try to rotate your topics that you choses to read out. When u find it hard to focus. Start writing what you are reading.I mean you can even do exact copy of what u r trying to read.
try to finish at least one topic with this technique and then give urself a reward by taking quick 5 minutes break. Or anything u would like to do. Come back and try to recall what u have finished previously.
keep an empty box. Write every thing ie date, topic etc which u tend to forget on small piece of paper, fold it and then keep it in ur empty box.
Anytime of day when u find concentration problem, blindly pick out any topic from the box, focus on it and recall what you can remember about the topic. It will give you kind of memory-shock therapy .
Pretty soon u will find improvements in your concentration ability and Memory. Inshallah