Depending on relationship, sometimes it is impossible to forget.  I lost my only child to cancer, telling me to forget her is like telling the sky not to turn blue.  Impossible!  When you are in so much pain because you can not forget.....turn to your inner strength and your faith. Thank God/Allah or what ever higher power you believe in that you at least had the time you did with that person, because though you may not see it now, you learned you are stronger than you thought and in time you will see, if this person left you, they were not good enough for you from the beginning.   :)