MacOS – Unable to unlock System Preferences in High Sierra

First, setting up separate admin and standard accounts was NOT a mistake. It is, in fact, a Good Idea™.

When you need to authenticate (for example, to unlock a padlock), you need to enter both your admin name and your admin password. If you're in your admin account, the name will be filled in for you. If you're in your standard account, both the name and password fields will be blank, and you need to enter them both. Be sure to enter the admin's name and the admin's password.

Your description is a bit confused, so I'm not sure exactly what's going on. The current version of FileVault (technically FileVault 2) encrypts the entire disk, not individual accounts. It's normally set up so that any of the users' passwords can be used to unlock the volume at startup time. If you've just lost your user password on a FileVault-protected volume, but remember the password for an admin account on the same volume, just start the computer, log in to the admin account, then use the Users & Groups pane of System Preferences to reset the other user's password.

Note that after you do this, the user account's keychain (an encrypted password database stored within the account) will still be encrypted by the account's old password. When you log in to the user account after resetting its password, you'll get an error stating that the system was unable to unlock your keychain. If you don't know the old password, there's no way to regain access to the old keychain, so the only real choice is to select "Create New Keychain", which discards the old database and starts over with a new (blank) password database.

If that's not what the problem is, then please clarify what's going on.