I have a hard time finding fault with Bella because I can't think of what she might have done differently that would have caused anyone any less pain. Given that she was unconditionally and irrevocably in love with Edward, she was going to forgive him when he came around. Once she forgave Edward, hurting Jacob was inevitable. It was only a question of when and how badly. She had always been completely honest with Jacob about her feelings or lack of feelings for him. (At least she was as honest with him as she was with herself.) She would have walked away several times if she wasn't sure that doing so would have hurt Jacob even more. After the pain that she had endured when Edward just walked away from her, I don't think that she was capable of doing that to somebody else. You can say that she should have let him go, but Jacob was holding onto her even more than she was holding onto him. Even her father is telling her that Jacob would rather be friends than nothing. Not to mention that if she were sucessfull in walking away from Jacob, it would have thrown a monkey wrench into the plot and caused the death of one or more of the characters.
I think that a lot of Jacob's pain was self inflicted. He knew that Bella was totally in love with Edward, but he kept trying to win her affections anyway. I also think that what hurt Jacob the most was Bella's decision to become a Vampire. In other words, I don't think that it would have hurt Jacob as much if she had married Mike Newton because at least she would have been alive if she married him. If she had been sucessful in walking away from Jacob, I don't think that he would have ever become reconciled to that fact. When she was changed, he would have mourned her as if she had died. I believe that Bella stumbled upon the path of least pain. Because she was unable to walk away from Jacob, he was able to hang around long enough to be reconciled to her decision to become a Vampire, and he found that Vampire Bella was still Bella after all. In the end everyone was happy, and this happy ending could have only been obtained through the treacherous waters of pain that Bella was somehow able to navigate.
I don't feel sorry for Edward's pain because I feel he earned every bit of it.
I know that Jacob claims that he was the natural course that her life would have taken. That doesn't feel right to me because it was Edward that brought her to Jacob in the first place. It was a desire to learn more about the Cullens that prompted Bella to flirt with Jacob in Twilight. If she hadn't been motivated by her desire to unlock Edward's secret she probably won't have paid much attention to Jacob at all. At that point she felt that he was too young for her. Also her plan was to go to college, some place sunny, start a career, and then get involved in a serious relationship. It was a desire to get the motor cycles fixed and hear Edward's voice again that prompted Bella to go see Jacob in New Moon. Given this, I don't think that Bella would have noticed Jacob at all if it hadn't been for Edward. Without Edward, she would have completed her high school education, gone to college someplace sunny, and never stepped foot in Forks again. She really didn't like Forks and had been vacationing with Charlie in California for the past several years. Only Edward could have gotten Bella over her distaste for that small rainy town.
Bella tried on several occasions to express to Edward the intensity of her love for him. The problem is that Edward does not believe that a human is capable of the same kind of love that a Vampire feels. And for most human's he is probably right. Think about it, over half of human marriages end in divorse; however, all of the vampire marriages that we see are still going strong even after several human life times. Edward can compare the intensity of the love between Carlisle and Esme, Emmett and Rosalie, and Jasper and Alice to any human couple and see first hand how much weaker human attachments really are. For us it is easy to see how completely and hopelessly in love Bella is because we live in her head. Edward cannot get into her head, so he never sees what we know so clearly. I don't believe that Bella could have convinced Edward of the true breadth and depth of her feelings until she found a way to let Edward into her head. You would have thought that Edward could have used Jasper's ability to feel what she felt to better understand the true nature of her feelings, but I don't think he would let himself believe that she really loved him as much as he loved her. I think Jasper understood and that is probably why he was able to say with such confidence that she was worth it. Of course, that could also have been motivated by knowing how much Alice loved her.