If Edward were more perceptive of his own emotions and condition, he would recognize a number of red flags in his relationship with Bella.
It was partially not his fault.
He had only been 17 when Carlisle turned him, so his mental and emotional development ended at that age. He was forever frozen with the incomplete brain development of a teenager, despite all his years as an undead creature.
He was also distracted by the very real red flags in their relationship in terms of him being a predator and her being prey. That part he understood fully was a real problem.
She had of course proposed a solution to this, turning her. Then they would be the same species at least and the constant threat of himself or another vampire killing her would be gone.
His disgust with himself and the desire for the many things he had missed out on himself kept him from agreeing. In all his teenage wisdom, he would rather she grow and develop normally and enjoy all the things he missed out on than have a partner in the same developmental bracket as him.
In a weird, sort-of self-sacrificing way, he knew there was a possibility of her growing up and leaving him because he never could, and he was willing to let her go if that happened. He thought it an unlikely outcome, but one where he would gladly sacrifice his happiness for hers.
The truth was that the red flags were not just skin deep, pun intended.
Because she was parentalized and he had a century of life experience, they had found a connection in each other that they could not find in their other classmates.
A child who endures parental inversion feels and acts more like an adult than a child and finds difficulty in bonding with other children who feel and act like children. A teenager who can never finish normal brain development but who is subjected to years of painful and real-life experiences develops a similar condition.
Their loneliness drew them together and kept them together when they might have needed some space to work on themselves.
All of that went right over Edward's head, and his focus remained on her physical predicament.
There were many nights since they began dating that he sat in his room and brooded, even more so after the incident with James, thinking of all the danger she was in because of what he was.
With all of this working against them, it really should have surprised no one what happened during and after Bella's birthday party at the Cullen house.
There was one out of place thing that nearly disrupted the outcome of that day, and it started a week prior to that incident.
New students were rare in Forks. His family was the first in a few years to show up. After them came Bella, and even that was about a year later.
Edward was immensely surprised when he arrived one morning to see a brand-new car parked in a formally vacant spot.
He listened to actual conversation and private thoughts as he, Bella, Emmett, Rosalie, Alice, and Jasper walked in to school.
Two new juniors had started, but most students knew very little about them.
There were tons of theories, but very little he could call fact.
The very first thing that Edward had noticed about Bella was her lack of thoughts. It was radio silence from her head. The very next thing was her incredibly appealing blood.
The first thing Edward noticed from the new students was their difficult to read thoughts. He noticed it first with the girl. Her thoughts were coming through like a phone call with bad reception; he was only getting every few words clearly.
Surprised, he flickered his attention over to the boy…and heard elevator music. Only elevator music.
Blinking faster than he should have from surprise, he tried to listen to both of them the rest of the day.
When he did get close enough to smell them through the rest of the building, he was surprised once more. They didn't quite smell right.
His first thought had been to wonder if they were from the reservation or at least related to any of the shapeshifting wolves he knew of. They smelled wrong too.
He shook it off because they didn't smell like the wolves had, but they didn't smell like the normal human should.
He made it a careful point to surveil them, and did his best not to tip Bella off. He followed them home more than once, checking out their story and snooping around on a day he missed and he knew they were at the school building.
He checked in only with Alice, who he knew would see what he was up to anyway, and she shared with him that all was well, nothing because of or involving these new students seemed to be a threat.
It was only because he had that week to make sure they were not a threat that he did not change his mind. Bella was not in any danger because of the new students, so he did not need to be there to protect her from them.
So when Jasper gave in to blood lust and Edward only hurt Bella further while trying to protect her from his out of control brother, he knew exactly how best to protect Bella.
He broke up with her, got his family to move out of town, and planned to live the rest of his life in isolation mourning his relationship with her that was over too soon.
As far as a seventeen year old's brain was concerned, this was the most logical and considerate thing to do.
He did not think anything of Bella's suffering, past the angsty thoughts of how she would be sad but sooner than later move on and enjoy life. He was sure the brief period of sadness was a small price to pay for the enormous benefits of a normal life she would get to enjoy.
He did not think anything of uprooting his family and interrupting their growing relationships with Bella. It was something they did all the time after all. They all knew that humans passed quickly while they continued on, so why should they miss Bella more than any other human that had gotten even a little close to them? Why would he think twice about asking them to do something so trivial?
What he did not stop to consider was how isolated and lonely Bella was before becoming close with him and his family. He did not think about how the fragile and shallow human friendships she had started to make her first year in Forks had withered from neglect because he and his family occupied most of her time.
After leaving Forks that night, when he left her alone in the woods, he was so sure he was right. She would be a little sad for a little while, but she would be fine.