Lily was important. She would never know how important; no one ever does. It takes at least a generation for that to settle out. But, by the time she had Harry behind her and Voldemort in front, she knew that her actions were the most important thing in the world. Whether they would be important to the entire world did not matter. He was her child. No matter what, Harry was the center of her universe. With James dead, in that moment of action, he was her whole universe.
The killing curse works quickly. She had no time to think. Even had she, she probably would not have lingered over that one memory. But still. It sits in his memory.
It was what he thought, when his head had not yet caught up with his heart, when his thoughts were still jumbled enough that, Lily is dead, so I must be dying, made sense.
"It is real, isn't it? It's not a joke? Petunia says you're lying to me. Petunia says there isn't a Hogwarts." Petunia was an idiot for suggesting such a thing, as if she could raise her station by upsetting her sister. "It is real, isn't it?"
"It's real for us." And Severus meant, That's all that matters. You are here with me. Lily heard it the simple way, he thinks, that they could go and Petunia could not, and that a place you cannot go is not 'real'. He meant some of that. Your sister cannot touch you in Hogwarts, not even with words. It will be our place.
He'd hoped for her love. But even when she picked the Potter boy, his love was hers. His head caught up. He was not dying. He considered trying to forget, but gave up on that fairly soon. Heart or head, it didn't matter. Heart could not give up on her, no more than he'd been able to the first time she was out of his reach, married to his bully.
And his head... Well. That was the only memory. She was his Patronus. If he lost that memory, he lost the spell.