You'll find often the problem is because you are getting back into the habit of studying after a long time. I've had this feeling many times. In fact, I now find if I don't study or read a book for more than a month I have the same problem again. I'm like you, get distracted to easily. I tried many things, increased the intake of fish, took Ginseng and other energy/motivation related supplements. Increased the fruit in take, but didn't really see much improvement.
For a few years I've wanted to have a dab at working on some kind of programming project which was related to aviation. I’m currently in IT, but not software programming so had a lot to learn. I knew deep inside I wanted to do it but couldn’t find the right moment to begin. Every time I tried I would just laze away from it. I felt I didn’t have enough time but this was obviously not true. Ironically, after the birth of my little girl I completed the said project within a few months. In fact said project was being worked on whilst mum was ironing and baby needed feeding – by me! What happened in that period? In lay terms: ‘The more your brain is forced to do, the easier it becomes to do more’.
What you have to realise is that the initial step is going to be difficult, but you have to stick at it and then you will find it much more easier. Step out of your routines a little and do things differently. There is a scientific basis for this. When you do something differently your brain has to learn the new steps. Inside your brain new electric paths are created which spurs on brain activity and gets you in the mood to do even more.