I have this same exact problem as well! That's why I hate reading cause I can't. But when I do read and drift off I try to catch myself as soon as possible, tell myself don't think of something else, then close my eyes and take a deep breath and start reading again. Just try not to worry and think so much while you're reading your book. And let your mind sink into whatever you are reading.

Hi, Did you hear about a psychological trick called "stay in the here and now"?!!! It means stay alert, don't allow your mind to go back in time remembering past events or think about tomorrow and what will happen then. To achieve this you need to: 1- Stop thinking of anything but the moment you are living . 2- If you ask me how to do it!!! .Simply use your five sense to know and feel everything around you. 3- Say in a loud voice: What do I see now? / What do I smell now? /What I hear now? /What I feel now? Where am I now? /What is the time now? 4- After answering these questions in detail you will be fully attentive and you'll escape any state of mind-absence. 5- Try to keep this state of attentiveness as much time as you could. 6- Doing this excersise regularly will make you get used to this state of attentiveness and in time you will be able to do it easier than first time. 7- I assure you that after a short period of time, you'll be able to be fully attentive and able to focus on whatever you are doing and may be one day you will miss moments of mind-absence :D P.S. I read about these tips and tried them myself they are really effective. They just need patience and practice to move from the stage of training to the stage of a habit. Once attentiveness is one of your habits, your goal is achieved. Good luck,

Try playing with jigsaw puzzle or learn to play chess. These are activities that helps train concentration.