How to turn off FileVault whilst loosing administrator and password privileges?

Thanks. OK, your computer does not have a T2 chip, so the routine I posted should work.

Sort of confusing but I think you have lost Admin status on your user account. Am I correct? Does your Mac have a T2 chip? What Mac model do you have? To find the complete model information select About This Mac from the Apple menu. If your Mac does not have a T2 chip, then you can try this:

Accidentally Changed Admin Account to standard on Mac

If you see your user account in your Mac laptop accidentally changed from Admin account to standard then you have to create another account and make it as an Admin account after that you can change your old account to admin again through the new admin account. Don't worry, you will understand once you follow the steps here.

Please follow the following steps carefully:

1. Restart your Mac ( via Apple Menu —> Restart)
2. Press and Hold both “Command + S” keys as soon as you hear the startup chime sound.
3. Leave the keys when the text appears (in some cases: you will be asked to enter your current user’s password after restarting your computer, after you enter the password then you will be in terminal mode).
4. Once you are in terminal mode, type the following commands carefully and hit RETURN after each command:

Type  /sbin/mount -uw / (there are two spaces here: first space between “/mount” and “-uw”, second space between “-uw” and “/ ”)

Type rm /var/db/.AppleSetupDone (there is only One space here: between “rm”  and  “/var/db/.AppleSetupDone” , no space after /db/. )

Type reboot

Now your Mac will reboot and you will be asked to select a “language” for the new Admin Account (Don’t panic it is just a new profile for a new admin account and your data are still safe).

Follow other normal steps then once you’ve created a new Admin account and you are in the new desktop, go to System Preferences —> Users & Groups —> click on “Click the lock to make changes.” —> now select your old user account (The standard one), check the box where it says “Allow user to administer this computer” —> it will ask to reboot your Mac —> after rebooting the computer —> select the user which you changed to from standard to Admin (old one not the new fresh one).

If you want you can delete the new user (fresh one) and keep your old one.