Don't worry you can always "fix" cast iron. What you need to do is scrub it down real good so that all of the rust is gone. Then start breaking it in all over again.
Keep the griddle oiled and then anytime you use the oven throw the griddle in there after you finish cooking with a good coat of oil in it. I'd let it sit in the oven while it cools off. When I mess up my frying pans I try to cook things that won't stick -- like sauteing onions or cooking fries (back in the days when they were really fried

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Just take it slow, it really takes months and months to break in cast iron well. I try not to wash with soap and water unless I have to. Whenever possible I just wipe it down with a paper towel.
It would also help to keep a thin coat of oil on your cast iron after washing it.
I love my cast iron and find that after a while (like a year or more) that it can be almost as stick free as some of the coated pans. Good luck
Pada