[CupcakeDolly]

Once again, Suzerain has the best advice. Some parts don't seem to be as significant to the story or characterization and go on for longer than they need to, and it has a kind of choppy flow that could be easily fixed by merging shorter sentences together. And like Suzerain said, "showing" what happens in the story has a better effect than straight-up telling it. It seems like Natalie finding out that Mark had been looking for somebody else is supposed to be your climax, so you should definitely put more emphasis on it.

As for the ending, I'd personally like to see Natalie moving on and finding happiness while Mark goes on to be alone and miserable, but that might be something that's been done too many times before. Of course it's always up to you. =P