How to mend a broken heart?

An Age Old Question. The topic of many poems, sonnets, and songs, to which there is no easy answer. Try to occupy your life with the good things-friends, activities, and other healthy endeavors. A broken heart heals in time.
Time Heals. It is better to have loved & lost than never to have loved at all. Remember the positive things about the relationship, these beautiful fragments of experience that now are a part of you. This presumably has made you better. Perhaps the learning from this experience will make your next relationship all the better. But don't rush it --- it is appropriate to mourn the loss of a relationship.