The ability to concentrate depends on adequate sleep, decent nutrition, and the increase in well-being that comes with exercise. Create interest in the material and a sense of purpose to the task. Human beings are bored by what is not relevant to them. In addition, it always helps to sit down to work with a clearly defined purpose and task. Establish rewards for accomplishments.
Human beings work best with positive reinforcement.
Start a worry book if frequent worrying undermines you. Worrying deals with issues of the past or future, and studying requires that you be clearly focused on the present. Getting down to work is the difficult part.