Do you have an email address that may be similar to others?
My work recently implemented a login process for customers and it was surprising how many user errors we had related to names/emails that were similar (so bob@gmail.com vs nob@gmail.com etc - these were not the actual addresses but you get the idea).
There's your problem. If autocomplete didn't exist I'd go nuts having to type my email address, and clearly those people don't have autocomplete or they wouldn't be making mistakes. Just use usernames.
Then nobody remembers their user name, I don't remember any of mine. If I didn't have LastPass I'd be password resetting on every financial account I have.
My work recently implemented a login process for customers and it was surprising how many user errors we had related to names/emails that were similar (so bob@gmail.com vs nob@gmail.com etc - these were not the actual addresses but you get the idea).