Any way to undo new account creation? (lost password)

I don't know my admin/login password and I followed these steps to create a new account:
Change Mac admin password without the disk [Solved]

I didn't realize all my files would become unaccessible. Now I can't log in to my old account to get any of my files, and I really need them. Is there any way to undo what I've done? I really need those files.

The procedure clearly warns you in bold print that you will lose access to your files. Why did you go ahead with it anyway?

Try this from Apple: Enabling the Root user

Once you enable the root user, log on as root. The root user has access to all accounts. That will allow you to move all your files to your new account after which the old one can be removed. Just be careful while you're logged on as root and once everything is back to normal, only log on with your new administrator account.

Or if you have backup by way of a clone to an external hard drive, boot from that drive.