Many years had passed since that horrible day at Godric's Hallow. Fifteen, in fact, each day as painful as the first to Severus. But Lily, his Lily, and that horrible Potter lived on. The eyes of his one love entered the classroom every day, eyes that sent jolts through his chest. But the body surrounding the eyes was worse torture to him. Potter, not Evans. It would have been better if the child had looked more like Lily. It may have been bearable then. But it was Lily's eyes in Potter's body. A reminder that she had been with him. Not Snape.
Severus saw them daily, the students. The ones that were in love as he had once been, still was. Potter and Weasley among them. Every time he saw them, he was brought back in time, and each time, he had a harder time keeping the memories down and out of sight. Another reminder of his past. Another fact he couldn't escape. The couple looked exactly like the pairing he despised, the pairing he had wished to modify, getting rid of Potter, keeping Lily.
The Weasley girl looked exactly like her, except the eyes. Potter's kid, no, Lily's child, looked like his mortal enemy, except the eyes. Seeing them together almost broke him. If the two's eyes were reversed, the pairing would be exact. Yet another thing that brought him back in time every time he entered the hallways of the school. He was spared in Potions, where only students of the same age took his class, and he was only plagued by one memory. The eyes. The rest of the time, he was forced to watch Lily and Potter walk through the halls of Hogwarts once again, himself an unimportant bystander, like before. However, this time, he didn't have the choice to walk up to her and apologize, didn't have the chance to hex Potter for all he was worth. They were dead, and their clones didn't deserve his wrath. His emotions. His sorrow. They were innocent, except for the crime of reminding him of his life so many years ago, the life that had finished when they got married and were killed.
So he was forced to watch Lily and Potter every day, only sometimes using Occlumency to push the memories down, sometimes not even realizing he was daydreaming. Seeing them, he often visualized the Weasley girl with green eyes and himself next to her, wishing for a girl that was gone, a time that had passed, and would be gone forevermore.