There was no hope to begin with.
Long before entering the house the Potters had hidden in, Snape had known that Lily was dead. He knew from the very second the Dark Lord had recited the spell.
Snape hadn't been there; he hadn't seen, or heard, or even exactly known what had taken place. But he had felt it.
The feeling came over him abruptly and instantly gave him the sense that something was terribly wrong. The feeling was truthfully the worst he had ever felt - as if someone had taken a wrench to his heart and twisted it, ripping the organ right out of his chest. And in truth, it had taken his heart, leaving him as nothing but a shell - a cold body that knew it shouldn't still be alive.
"Lily."
It was automatic; he wasn't even thinking when it suddenly escaped his lips. But, as soon as he whispered her name, he knew.
There was no hope to begin with.
That had only made the pain worse.