Unable to reset administrator password

I have tried and tried aaaaand tried to RESET my imac administrator password to no avail.

Going through system pref/Users & Groups does not even give the option to reset a password, only to change it...which requires you to know the previous one.

I am trying to make changes (i.e. adding external hd's) and the box pops up saying I need to enter my password. Problem is, I have lost it in my old phone that was destroyed.

I have checked everything on this link: https://support.apple.com/en-us/HT202860 and nothing helps! I can't reset the password at the login screen because I do not HAVE a login screen. I cannot turn the login screen option on because you have to have the administrator password!

I'm so frustrated and begging for help. Please, please help me. If there was a 1-800 # I would call it!