You should know this: the further the shadow goes from the object the more it loses contrast and blurs a little bit.



Load the selection in Select > Load selection > Alpha 1



And transform a little bit the selection.



And apply blur or blur more filter.

This Filter will five you blur in the end. But I think every stilling should be done with hand.

So shoes a nice SOFT brush, set the flow to about 5-10%, and paint a little bit with mere white.







And there we have it.