Locked my sa user out
There are two ways to handle this.
One option is to login to the windows machine as a local administrator, start SQL Server in single user mode (-m startup switch) and you will now be able to get into your SQL server as a sysadmin using Windows authentication.
The other option is to quite simply change back to mixed mode by modifying the registry in windows directly (not supported). On my machine, the registry path for a named instance (named "a") is: Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL14.A\MSSQLServer. And the value for mixed authentication is LoginMode: 2.