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.
In many fanfictions, Bella initially tries to be strong but soon crumbles. She may cry a lot and have trouble moving on. Some stories might show her leaning on her friends for support, while others could depict her trying to find a new purpose in life without Edward. She might also be angry at Edward for betraying her love and at Tanya for 'taking' Edward away from her.
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.
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.
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 He could become withdrawn and moody, similar to how he was before Bella entered his life.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 He could become withdrawn and moody, brooding over what went wrong.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.
Well, at first, Edward would be in shock. He wouldn't believe that Bella actually left. Then, he would likely start to search for her frantically. He might go to all the places they used to go together, hoping to find her. His reaction could also be one of self - blame. He would think that it was all his fault, that he didn't protect her well enough or love her the right way. As time passes, he could become more resigned to the situation. But he would never really forget her. He might even try to keep an eye on her from a distance, just to make sure she's okay, even though she left him.
Bella would be heartbroken. She has put so much trust in Edward, and this act of cheating would make her question everything. She might become angry and lash out at both Edward and Tanya. She could also become very sad and isolate herself from others.
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.
Bella might become more independent. She could start to rely on her human friends and family more, like Charlie. She would have to face the challenges of pregnancy without Edward's support, which could make her stronger in the long run.
Edward would probably be very confused at first. He would try to understand why Bella would leave him for Rosalie. He might start to analyze all their past interactions, looking for signs that he missed. Eventually, he could become angry, not at Bella directly, but at the situation. He might also feel a sense of betrayal, not just from Bella but also from Rosalie in a way, since Rosalie is 'taking' Bella away from him.