OP wrote:
"I’m one of those that unfortunately don’t have a Time Machine back up."
Lesseeee here....
No backups?
And you tried new, totally beta software that was unproven on your main machine?
And now, you want to go back?
Ahem.... isn't that what "backups" are for?
I suggest you download CarbonCopyCloner from here:
http://www.bombich.com/download.html
CCC is
FREE to download, and it's
FREE to use for 30 days.
Next, connect an external drive to the Mac.
Use CCC to create a clone of the internal drive on the external drive. Easy enough.
NOW, try the internet recovery process outlined by chrfr above.
I suggest that you run Disk Utility first, and completely re-initialize the internal drive.
Then, do a clean install onto it.
After the install is done, before you reboot "into it", connect the CCC cloned backup.
Now reboot and go through the initial setup sequence.
At the appropriate moment, the Setup Assistant will ask if you want to migrate data from another computer or drive.
"Aim it" at the cloned backup.
Choose to migrate apps, accounts.
I might leave "settings" UNchecked, because preference files created in High Sierra might not work in [regular] Sierra.
Bring over data, too.
That oughta do it.