I am torn here, folks. I have lived through a time without the other piece of my soul. When he was gone, I can honestly say that it was my kids, having to get up every day and take care of them, keep life some semblance of normal for them, that kept me a form functional. But, I knew (for the most part) that he was coming back. I (thankfully) can only imagine that, if that weren't true, it would be similar. I would endure because my kids needed me. How could Bella not do the same for the daughter, Edward's daughter, that she faught so hard for in the first place?
At the same time, this is where we have to seperate real life from the mystical in the story. Bella, at the time, was not only resolving herself to most assuredly loosing (most likely) her soul mate as well as her entire family, but also the idea that she had a very slim hope of giving her daughter a chance to live. Bella was a key to the battle, even if she didn't realize that she was the key. By staying and fighting and therefore dying, she would give Renesmee and Jacob a chance to live. It was choice, yes. But, it was the choice between running carelessly to die and dying so that someone else can live.
But in real life you wouldn't have someone to take care of your kids forever. I think if she were to leave Nessie with Rose or Alice or something like that, it would be bad. But Nessie had Jake forever and he was everything she needed forever. I think she felt that was better. Plus, she, unlike Edward never really was suicidal. She didn't think she would want to go on without Edward and had said so in the past, but never acted on it. I think if the battle had been bad and she lost Edward, she would have went to find Jake and Nessie.