Because Twilight is a romantic drama warped up in the setting of supernatural creatures, there are a lot of sad things that happen to the main characters. While other characters, including some other members of the Cullen family, might have things worse than Edward, there are still some sad things that happen to him throughout the series.
This makes a lot of sense, as he is the main male lead in the story, and there has to be conflict and drama with him to make him an interesting character. There are some unfortunate things that happened to Edward in his past as well as his present.
He was without a partner for many years
While being without a romantic partner isn’t always a sad thing, it can be if it’s something someone wants.
Edward was the only one in his adopted family that didn’t have a partner of his own, so he was basically the seventh wheel for decades. This seemed to bother him somewhat and made him feel lonely, and he didn’t think he’d ever fall for anyone.
He had to watch Bella fall in love with Jacob
There were times when Edward was too controlling of Bella, but, in other ways, he was understanding of her affection and bond with Jacob.
He realized that she had feelings for him that were created after he left her, and he never really forced her to get rid of those feelings. But, it was still hard for him to feel like she was torn between them.
He wanted to die when he thought Bella did
The end of New Moon is very reminiscent of Romeo and Juliet. Edward believes that Bella has died, and he decides he can’t live in a world without her.
So, he plans to do something extreme so that the Volturi have to kill him, as this is one of few ways a vampire can die. While he was a little rash in his thinking, it’s still tragic that he felt this was the only option he had.
Both his parents died
While readers don’t hear a lot about Edward’s backstory in the original saga, more information has been given by Stephenie Meyer and also in Midnight Sun.
While Edward wasn’t particularly close to his father, he had a comfortable and happy life and a close bond with his mother. However, this changed when all three of them got the Spanish flu. His father died first and then his mother.
He didn’t always feel like he belonged with the Cullens
Edward definitely doesn’t always seem to connect with others around him. He does love and care about the members of his family, but he always felt like the odd one out.
He was prone to loneliness and melancholy, and this made it difficult for him to find emotional peace. He was definitely more introverted and spent a lot of time hating himself.
Because he lived as a vigilante
This is a bit of a complicated point, because the morality of what Edward did is debatable. He had the ability to read minds, so he briefly went through a phase of drinking human blood.
His justification for this was that he would only kill humans who were monstrous and going to do awful things to others, but this period of his life was definitely hard on him.
He almost lost Bella many times
The person that Edward loved most in the world was Bella. He fell hard for her, and he always wanted to protect her. Of course, this doesn't excuse many of his problematic behaviors, but it’s not as if the story is supposed to be completely realistic.
The times when he was most upset was when Bella almost died. This happened multiple times, including when James nearly killed her.
Because he blamed himself for ruining Bella’s life
Edward had a lot of guilt about falling in love with Bella. He didn’t want to ruin her life and take away her ability to grow old and be human.
He didn’t want her to have to leave all of her family and friends behind either. He also had to go through the anguish of seeing Bella nearly die while she was pregnant, and he definitely blamed himself for this even though he didn’t even know it was something that was physically possible.
He believed that he was damned
The big reason why Edward didn’t want Bella to become a vampire is that he believed that becoming a vampire made someone damned.
He did believe in souls, but, in his mind, a person who became a vampire would be a monster and have no option of an afterlife. This just showed the depth of his self-loathing, and it’s pretty tragic.
He was dying from the Spanish Flu at 17
Edward was luckier than most people, as he had a well-off family and parents who did care for him. He had a promising life ahead of him in many ways, but this all changed when he was dying of the flu during the pandemic.
He would have died if Carlisle hadn’t turned him, and while he did get to have many good experiences because he lived on as a vampire, he often wished he had never been turned.