Re: fixing what was said in a rejection letter and resubmitting:
Don't assume that the brief comments in a rejection letter can possibly contain every item that caused the piece to be rejected. Even a short story might have 5 or 10 major faults -- if an editor uses one or two as an example, and you fix those two, there may still be other things the editor didn't like.
Plus, seeing a manuscript a second time is tricky -- unless you get a different reader, they'll probably just think "This thing again?" It's hard to make it fresh the first time, much less the second time around. Just send the work somewhere else. Different editors and readers will comment on different things, and it's up to you to figure out which changes (if any) to make.