I had this error this morning, but I found the cause; see my answer below.
certbot.errors.PluginError: Unable to determine zone_id for <my_new.domain> using zone names: ['my_new', 'domain']. Please confirm that the domain name has been entered correctly and is already associated with the supplied Cloudflare account.
That means you've authenticated to cloudflare but cloudfare won't let you edit the zone.
The fix is probably to look at your tokens at https://dash.cloudflare.com/profile/api-tokens and make sure you've set it to allow edit of all zones, or at least the one you're trying to use with certbot.
Sorry, I wasn't asking for help. I gave the answer in my message. I searched the site and the net and didn't see the question answered but I thought it would be valuable to document it. If I'm the only one, feel free to delete this thread.
Ah, sorry, I thought the lower parts of the post were also part of the error message. My bad. To make your post more clear, you can edit it and put the actual output in "Preformatted text" format (the </> symbol in the post editor, or select the output and press Ctrl-E.