If there's one thing you should know about Peter Parker, it's that he really did love Gwen Stacy. And not in the high school sort of "I can't bear to not see you until lunch" kind of infatuation, but a real, genuine love that he planned to foster for the rest of his life.
When she died, it felt like his heart was ripped out of his chest, still beating for her. He can't stop hearing the sound of her head hitting the ground; it resounds through his mind like a nightmare.
He doesn't move on, at first. He visits her grave, spends more time there than at his own house. After that, he throws himself into Spiderman, taking out his guilt and aggression on New York's finest criminals. But even when putting himself in life threatening danger, he can't stop hearing that awful crack, can't stop repeating you killed her, you killed her, you killed her.
It's not until the new neighbors move in, and he meets the spitfire redhead that is Mary Jane Watson, that he starts to get better. Face it, Tiger. You just hit the jackpot, she says, and after a few weeks he starts to believe her. She puts him back together piece by piece, and soon he stops hearing Gwen's death, stops having nightmares of the fear in her eyes as she fell.
He never forgets, though. He can feel the pull towards MJ, he knows he could fall hard, but he also knows that he'll love Gwen for the rest of his life. But maybe, he thinks, it's possible to love two people at once.
So he gives himself over to her, lets her fix his broken self. He starts smiling again, goes to Gwen's grave less often, and stops beating up every criminal in sight. Instead, he spends his time falling for the fiery haired Mary Jane.
But even then, he still has a responsibility (With great power, he thinks). He is Spiderman; he can't give that up, and the city of New York needs saving. Because, for some godforsaken reason, there never seems to be a shortage of criminals with weird backstories and crime themes in his life. But hey, at least he gets some prime pun practice.
He doesn't see this as an issue until one day, as he runs in late to a coffee date with MJ with a split lip and dirt on his face, that it all changes. It's a simple question, one that he should've seen coming, but it throws him off guard nonetheless.
"Where are you always running off to?"
And then it all comes flooding back. Gwen falling, fearfully reaching out to him in the belief that he would be able to save her. The snap of her neck on the concrete, her body sickeningly limp in his arms. She would've never been in danger if he hadn't dated her, if she hadn't made the mistake of falling for "the Spiderman."
He looks at MJ, this amazingly strong woman who has helped him recover through sheer power of will, this person he cares immensely about (maybe even loves), and one thing becomes clear.
She can never know.