how to stay focussed while studying...

There is a 4C method that most Psychologists trust by.

Concentrate: Concentration can be disrupted by Anxiety, fatigue, stress, your response tp a task at hand and your internal thought process. A good way to build concentration and maintain focus is to set process goals for each activity.

Confidence: Confidence in thinking right and aiming right. You can only be confident if you believe you can irrespective of the external factors. Confidence can be identified through your level of enthusiasm. You can develop confidence by imagining prior situations where the circumstances were similar but emerged successfully.

Control: Controlling negative thoughts , feelings and emotions from overtaking your mind is essential. Only allowing positivity is important.

Committment: Commitment to your goals is what determines your success to a certain level. Goals need to be short term and long term.

So everytime you want to sit for studying, try following these. Start slow and small hours of study and gradually move it to longer hours.