Ookay.. why are you disappointed in yourself? 0_o Because you didn't knock her up? Or because you don't want to be scared of it?
In any case... nothing you can do, really. If it says negative, she's probably not pregnant. Depends on how long it was since you last had sex. I'd wait a couple more weeks and have her take another one. But girls miss their periods all the time. Try not to be too worried... or hopeful? Not quite sure what you meant there...