You are having obesessive and compulsive thoughts recurrently. This is quite common in this age. You need to focus on your goal and be busy with your activities, like playing, sport activities, etc. And become relaxed to concentrate on studing, making notes, etc.
Start with homoeopathic medicines to treat this --
Take medicines Calcarea carb 30 Pills and Ignatia 200 Pills.
Start 4 pills each medicine x 3 times before meals x 30 days.
This might help.
Next Steps
take these homoeopathic medicines regularly.
Health Tips
keep a proper balance on life managing studying and outdoor games. keep fit and exercise for 30 minutes reqularly.