One thing that's helped me is to be disciplined-- as in very disciplined. Treat your study schedule like a job-- and every time you miss studying, or make a last minute change, impose time on yourself to catch up on studying. Reward yourself from studying by putting in a small amount of money into a piggy bank, like $1-2 each time you successfully stay focused and devote the time. At the end of term, reward yourself with the money by getting a special treat. I find when I reward myself for my hard work, it becomes easier to concentrate because I become fine-focused.

I'd like to ask, are you currently experiencing depressive symptoms? If so, it could be a reason why it's hard for you to concentrate.