Is an amended form the only way to make a correction if I haven't mailed my taxes yet?

[DoninGA]

No, you do not amend a tax return that has not been filed.

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state.  See screenshot

This will take you back to the 2018 online tax return where you can make changes to the tax return before filing.