And If Dawn had died a natural death, I'm sure Buffy would have moved on. That's grief, after all. But If Dawn was forced to sacrifice herself, that would make Buffy feel as though she had failed her. I'm not saying that it would have been Buffy's fault, but she'd feel as though it was anyway. Buffy has always had a tendency to blame herself when something goes wrong and I don't this case would be an exception. In Weight of the World, she blames herself for the fact that Dawn could die and even goes as far to say that she killed Dawn herself. If Dawn died it wouldn't just be grief, it would be guilt too.
Yes, Buffy did get over heaven, but that was because of Dawn as well. In the season six finale, Dawn is the reason Buffy keeps fighting and the reason she decides to live again.