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Here's a simple thing you can try.

Completely remove the background in the girl layer including the shadow.

Put the girl in front of your background.

Duplicate the girl, rename the layer shadow and with levels make it black.

You'll see specks of color, paint them black.

Blur the the layer, lower the opacity.

 

Move the shadow around and make sure the layer is in the right order so the shadow is behind the girl.

Use transform, add more blur if needed and adjust opacity until it looks right.