People think about these things in terms of 'emotions', but when you fall in love you become neurologically connected with that person chemically. Her body....her touch....her smells, what makes her laugh or pissed. You even become connected to her bullshyt, and dumb ass ways. You become one with her.
Im assuming she was prolly the first one you truly fell in love with. Its why mankind has always put an emphasis on your "first love". This person is literally the only frame of reference you have with loving another woman outside of your mother or family.
It takes time to my man...its like if you're driving a car, and you look in the rear view mirror. The object you're looking at becomes smaller and smaller quickly due to the speed of the vehicle. Because your mind doesnt want to rid yourself of your ex, mentally the car is inching along. The object (your ex) doesnt get smaller. Your brain doesnt want to lose sight of her.
Just keep dating and fukkin, and living an adjusted and healthy life...time heals. You'll get over the hump.