Neither the script, nor the Salvatore novelization offer any detail on why she thought she failed.
Being a part of the plot seems way too outlandish to pass Occam's Razor (and would have definitely been mentioned explicitly in canon, which it wasn't). So most likely, a more mundane explanation is one of the 2:
She felt she failed to prevent the explosion as part of Padme's security detail. Padme wasn't hurt, but at least 6 other people were killed AFAIR, including another handmaiden.
Her death prevented her from continuing to be an effective decoy and thus made the Senator vulnerable.
Neither one is confirmed in canon, but both at least make plausible sense from a point of view of a dying security decoy.
Out of universe, that was just one more pathetic line of dialog among many. We got a script full of Anakin's creepy come-ons to Padme, can't exactly expect Shakespeare-quality replicas here.