In the fanfiction, Bella could initially be in a state of panic. She might try to run away from Edward, thinking he has just doomed her. However, as the shock wears off, she might become curious. She may start to notice changes in herself, like enhanced senses, and this could lead to her having a more complex reaction where she is both scared and intrigued by what Edward has done.
She might initially be very quiet and withdrawn. Just sit there with tears in her eyes, not really saying much as she tries to process her emotions.
She might be devastated. Bella was deeply in love with Edward, so his leaving would be a huge blow. She could cry for days and feel like her world has ended.
Bella may react by trying to understand Edward's reasons. She could be more rational in some fanfictions and instead of just being sad or angry, she starts to investigate. Maybe she discovers that there are forces at work that she can fight against to get Edward back. Or she could decide that she doesn't need him and focus on her own growth and development, becoming a stronger and more independent person without him.
Often, Bella's reaction is one of shock and disbelief at first. She might not understand how Edward could leave her for Tanya. Then, she could either isolate herself from the vampire world completely or she might seek revenge in some extreme fanfiction scenarios. She may also start to question her own self - worth, thinking that she wasn't good enough for Edward.
She might be completely devastated. Bella has always been deeply in love with Edward, so this sudden abandonment could shatter her world. She could be left in a state of shock, unable to really process what has just happened.
He could be devastated. Edward is deeply in love with Bella, so he might go into a state of depression. He may isolate himself from his vampire family and friends, constantly replaying in his mind what went wrong in their relationship.
Edward might be initially shocked. He has always seen Bella as his own and this sudden change could be hard for him to accept. He could become withdrawn and moody, similar to how he was before Bella entered his life.
Edward might be initially shocked and heartbroken. He has been in love with Bella for a long time, so her leaving for Jasper would be a huge blow. He could become withdrawn and moody, brooding over what went wrong.
She could be heartbroken. After all, she's pregnant and the person she loves has left her. She might feel abandoned and alone, crying a lot and wondering why Edward would do such a thing.
Often, Bella reacts with intense heartbreak. She might isolate herself from her friends and family, feeling like she can't trust anyone anymore after Edward's betrayal.