So many people have brought tried to quantify and qualify different types of death. Biological death, textbook defifinitions of death, religious interpretations of death, social death, emotional death...This is a love story about two young people who are alive in all the ways that really matter: they laugh, and grieve, and quarrel, and kiss, and love each other so desperately that there is nothing they would not do for one another. Beside this, biological criteria aren't really very much to the point, though it's fun to think about.
On the other hand, the same with life. Other than the textbook definition, living means many different things to many different people. That's the beauty of it.
It's all in how you look at things. Death is also defined as the end. Or, if you're famliar with Tarot, a new beginning. If you look at these definitions, Bella is definititely going through a death of sorts.
But, I love how December has brought us back the story. What matters to the story? Is Bella mourning her homeostasis when she considers her rebirth into another life? I doubt it. That's not what I would be thinking about. I wouldn't be thinking about the phsyical. I'd be mourning the life I was leaving behind.
That's what is really dying here. Her former existence. Her old family is in a sense dying to her. Her new found friends will be dying to her. Her new best friend already considers her dead. The numerous beauties of human life will be dying to her.
Physically, she will be living on. But emotionally, a part of her will die as she is reborn in to her new life with Edward. She's sacrificing many for one. However, what she's facing is harder than outright dying IMO. When someone dies, it's understood. It's final. We even have definitions to help us understand and rationalize it. The life is celebrated with a wake of a funeral. People need to understand to move on.
What understanding will those Bella leaves behind find? That is what is weighing on her. The closest thing to closure that she could give her family was a wedding instead of a funeral. How strong the bond of love is between Edward and Bella that they can stand together side by side and choose this fate and all of its consequences for the promise of eternal love.