Maybe take a breather for a second. When you first set up WordPress, it emailed you a username and password. The default username is *always* “admin”. Now, you have the option – after initial login – to change the password. (As whoo said, the wp-config file has absolutely nothing whatsoever to do with your WP installation login.)
You also have the option, after initial login, to add a new user. With that, you can create your own username and password, and then set yourself to admin status and use *that* name to log in.
It sounds to me like, in the beginning, you set yourself up with a new username and password, then deleted the “admin” account. (Otherwise, the “auto” would recognize the username in the DB and send it to you – OR you initially used a different/old email address and it can’t find you that way, either.)
In any case, if you can’t log in, you’re going to have to reset. If you follow the last link Whoomai sent, you’ll see how to do it right there – there’s step-by-step instructions. We can’t get much more clear than that.