whatever reasons you had relation with her she has outgrown that relation ans she will never come back to you, Because you are expecting that one day she will come back and you are waiting for her that is why you are frustrated and follow her wherever she goes. Firstly divert yourself to something which is of more interest to you or if possible leave the place where you are staying change your job and place. There is a saying out of sight is out of mind, though it will be very difficult divert yourself to more interesting thing and just enjoy thinking of the moments whatever you had spend together. I will not tell you try forgetting because if you forcefully try to forget the frustration will increase , Only time and another woman in your life can bring the change, but whenever you feel lonely always you can remember the moment and enjoy that will bring solace to you.