Please, get off the "Drama Train", step away from the tracks, re-group, develop a pricing strategy, and start fresh.
If this cake is due in 2 DAYS and she still hasn't paid, I wouldn't have yet turned on my oven, so I wouldn't even be able to complete the cake--and I tell her that. It would be my reason for cancelling.
At this point, if you're really losing money on the deal, I'd cut my losses, go with her to WalMart and pay for the cake that she wants
and, NO, I'm not kidding. Better to give her a gift than to continue to kick yourself over YOUR OWN MISTAKES.
You can only get swept away with the tide if YOU step into the ocean.