I think I see where I went wrong. My diagram has 12 valence electrons for N, while I'm supposed to have only 8. But how do resonant structures have localized charges? Shouldn't they have delocalized charges?
No, delocalized charge is effect of switching between resonant strutures. Charge is localized in any individual structure, but as molecule switches between them very fast, what we observe is charge that is delocalized.
So, if I was asked to come up with a resonant structure for the nitrate ion, I would have to include all of the possible lewis dot diagrams for nitrate ions with a certain number of single or double bonds?