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.