You would need to move to a MySQL solution for authenticating your users. There are a few options out there for ULX and doing a search should yield you a result or two.
This will require some understanding of MySQL as well as how to use MySQL in php. You'll also have to make your loading screen a php file instead of an html file. You'll also need to use and have working the MySQLOO extension for gmod which you can probably find on Facepunch.