Why I Think Bella is Going to Have Accidents/Go Through a Dark Era
For what it's worth, this is not just my desire for Bella to through consequences post-Breaking Dawn. That being said, the character that Stephenie Meyer wrote has certain characteristics that make her prone to a reckoning one day. So, here are my bullet point thoughts in no particular order :
I think it's going to take a long time for the other shoe to drop. At least well over a century. Bella made a huge choice and staked her happiness and self-worth on becoming a vampire. This choice is validated by everything that happens in Breaking Dawn, and Bella has a long future ahead of her that involves Edward, college, watching their daughter grow up, etc.. Bella is going to be content, if not happy, with her choices for quite some time.
When the other shoe drops, I think it will have to be in the form of monotony. All vampires deal with boredom; Edward is literally staring at cracks in the ceiling as Midnight Sun opens. The world around them doesn't provide them much of a challenge, and they can't eat or sleep or do a whole lot to structure their time. So, boredom is coming for Bella eventually. One day she will have done everything and been everywhere. Nessie will have a life of her own, most likely. Her relationship with Edward and vampirism will become the status quo. The thing is, I think that monotony would be incredibly triggering for someone like Bella. When she became a vampire, she wasn't signing up to be bored for eternity. Oh, she'll deal with insatiable thirst and having to say goodbye to her loved ones for all she doesn't have to. But Bella is inexplicably drawn to the supernatural and the extraordinary.
Bella is internally quite critical of other humans throughout the series. Sometimes she makes comments about their appearance or behavior. I don't think this is because Bella hates other people or is particularly mean. I think average, every day, people are occasionally triggering to her, because they remind her of her own insecurities. She doesn't want to fit in with the boring human world when the supernatural world is right there. Bella resents normalcy (one of the reasons I think her and Jacob wouldn't have worked out. Jacob craves normalcy).
So what is a girl to do when 100 years have passed and everything she found extraordinary about the supernatural world has become normal for her?
Bella has no practice dealing with difficult things in a healthy way.
Eventually there is going to come a day when the only thing that would be new or exciting to her is the taste of human blood.
I don't think Bella has super-self-control. A big part of self-control is the psychological aspect. Anyone who has dealt with addiction or bad habits knows how much you have to believe in what you're doing. I think that Bella was just so satisfied with every other aspect of her life that giving up this one think was not that big a deal. Maybe she does have a talent for it, but she's never had to really work and earn her self-control. What happens when she is no longer feeling fulfilled by her vampire life? Chances are, her thirst could become a more persistent issue.
The only times Bella has to face struggles with her self-control are in two instances: her first hunt when she claims it could have been someone she knows (wtf, Bella?), and with Charlie, who she loves.
Bella doesn't connect well with other humans when she's a human, herself. She doesn't value humanity the way Carlisle or Rosalie do. Chances are, she's probably not going to be building close human friendships as time goes on.
The members of the family who have had accidents (Esme, Alice, Emmett, Jasper) are the ones more comfortable as vampires. By contrast, Carlisle, Rosalie and Edward believe there is something inherently wrong with vampirism that they have to overcome. Bella believes she was "born to be a vampire".
One day, all these points converge and lead to a breaking point. I definitely think there could be a situation well in the future when Bella is struggling with monotony on a deeper level and "accidentally" slips up and eats someone. She would probably feel conflicted, but accidents happen, right? But unless she starts to value humanity on a deeper level, her accidents might keep happening, which could lead to a family crisis. Obviously, the family holds an intervention and force her to confront her issues. But I think it would be out-of-this-world uncomfortable for Bella to face herself and her choices. She probably has an existential crisis. Within the context of a story with a purpose, I would use this as an opportunity for Bella to learn about how she deals with struggles. She has to realize that she isn't a burden and she needs to lean on her family and be honest when she's struggling.