i no longer have an admin account on high sierra

I tried to change my name in system preferences users & groups because my name has changed. After entering the new name, it went right back to the old name and suddenly made my user account a standard user instead of admin and now there is no admin on my MacBook at all.

I spent the last few hours looking through forums and even watching youtube videos on how to go into the system and fix it by restarting while holding command and S and entering code to remove the apple set up file to then reboot and create a new admin account but no matter what version of code I type in (found two versions), it says it is a read only file and just reboots with no change as my standard user account.

I read that with High Sierra and fire vault on, it does not work, and that happens to be the case for me. I am not very computer literate and now I'm pulling my hair out... can anyone help please? I seriously do not understand how just trying to change the name on the account could cause this problem.