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.
It could be that there are external forces at play. Maybe there are other characters or some kind of dark secret in the fanfiction lore that forces Edward to leave Bella while she's pregnant. For example, there could be a threat from a rival group that says they'll harm Bella and the baby if Edward stays, and he thinks leaving will draw the danger away from them.
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 He could become withdrawn and moody, similar to how he was before Bella entered his life.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 He could become withdrawn and moody, brooding over what went wrong.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.
One interesting fanfiction could be where Edward leaves Bella for a different reason than in the original story, like he believes he's putting her in danger from a new vampire threat. This could lead to Bella having to prove she can handle the situation on her own.
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.
One possible reason could be that Paul represents a different kind of life for Bella. Maybe he is more down - to - earth compared to the often brooding Edward. Paul might offer a simpler and more straightforward relationship without all the supernatural drama that comes with Edward.
One possible reason could be that Bella discovers a different side of Jasper that she finds more appealing. Maybe Jasper shows a deeper understanding of her in some aspects that Edward lacks. For example, Jasper's past experiences might make him more empathetic towards Bella's own struggles in a way that Edward, with his own set of vampire characteristics, doesn't quite get.
In some fanfictions, if Edward leaves Bella, she might become stronger and more independent. She could focus on her own life, perhaps pursue her studies or hobbies more intently. For example, she might finally decide to travel the world and discover new things on her own.