For starters accept the fact what happened. Is meaningless day after day and night after night to torment himself, asking why did it happen and what would happen if... Realize that your relationship is a thing of the past, and the sooner you mentally complete them, the faster your heart will go longing.
Do not communicate with your loved one, and especially do not look for occasions to meet, as if you wanted to. Erase his phone, tear up the letters, throw away your photos. Don't listen to the songs you listened to together. Music has a strong emotional impact, so if you're listening to songs reminding you of your former love, you will not be able to part with it.
Take a few minutes and think about the positive things that brought you a parting. For example, you have free time you can now spend on playing sports, a favorite hobby, socializing with those you love, and just for himself. Write a list of positive aspects on a piece of paper. If you approach this case with full dedication, by the end of the list you may be very happy that your love has come to its completion. Hang this list at the bed and see him at every opportunity.
Fill your day so that there is not a minute to be sad and a sad reflection on lost love. Head out into a career, Hobbies, socializing with friends. Try as little as possible to be alone.
Look at life with optimism. Teach yourself that the best in your life ahead, and true love will come. Look for the positives in everything that happens around you, watch good movies, socialize with positive people. Chase the sadness away, and very soon you yourself surprised notice that and forgot to think about a former love.