Hi, When you are thinking so much about someone, there are few intense emotions acting behind those thoughts. First step in the process is to identify those emotions and label them. Second step is to recognize different ways in which they have been affecting you far and try to channel them in different ways like rigorous physical activity, adventure or socializing. Second way would be to allow yourself to think freely and deeply till it stops making sense anymore and repeat it each time the rumination begins. Best option would be to consider counseling where you can be helped either to make sense & get rid of repetitive thoughts or to act on the them wherever relevant. Good Luck.