The current project is beating me up

I am trying to revise a short story that came back rejected. I got a personal reply from the editor, which I didn’t expect. Is it because the magazine in question is an online mag? Anyways, the editor said it had some interesting bits, but that one or two things confused him. SO…I’m working on clearing up the confusion. I see where he is coming from when I read the story again.

Trouble is: I am having a hard time subtracting words to tighten up the prose. The word count is 6000+ words. I have a feeling that I will add another 500 words to clarify the plot, but I don’t know if there is any fat to cut out.

How does one objectively decide if he or she has cut enough or if more cutting is possible?