I had this exact question, and according to our Nintex contact:

"Responsive forms aren’t available for Nintex Live just yet.  If you want to publish forms outside the organisation you’ll have to use the classic designer.  Unfortunately I don’t have a timeframe just yet as to when the responsive one will be made available for external forms.  The capability is already in place because that’s how Nintex Workflow Cloud operates but the missing link is getting the data from the form back to an on-prem environment."

Understood that Nintex Live is not yet ready, but then is there a way to switch back to Classic Forms?

Or do I need to create a new list..? I spent so so man times to make the list, and now you're saying I need to make a new one..?

Sorry, I found the 'Delete From' in the Ribbon.