Really assess the situation. Find as many reasons why you don't need to be together with that person. Remember and record as many of its drawbacks. Re-read the whole list. And that's the villain you are constantly thinking? Is it causes you to sleep? Pity myself. Think about what kind of person you certainly do not need.
Try to occupy every moment of their free time. Shopping, meeting with friends, reading books, watching movies. Anything but be alone with yourself and your thoughts. If possible, arrange for more work. It will distract you from sad thoughts, and even you will get tired twice as much, so at night you will sleep and not think about it.
Find a new hobby. It should be interesting and time-consuming. If it is summer, do some garden or vegetable garden, if winter start to ski or skate. Try to find a collective hobby, not to be long alone.
Every time you think about it, do something unpleasant. Go to wash dishes, to wipe a dust, to communicate with the unloved colleague or print the report. Another option – after each unwanted thoughts do 10 pushups. This will not only discourage thinking about him, but also will help to freshen the tone.
Start a new novel. They do say that a wedge a wedge knock out. If you will be nice man, you may not want to think about that bastard who "settled" in your mind. Try to communicate with men as much as possible. Compare it and those Cavaliers who surround you. Surely he will lose by many parameters.
Start to love and pamper yourself. Take care of your body: go to the gym, get a massage, visit a bath or sauna regularly make a beautiful hairstyle and manicure. Don't forget about regular shopping, because there is little that is uplifting as well as another purchased blouse or chic shoes with vertiginous heels.