Hello. I'm no digital deity, but I am a computer science major, and have been involved with data processing in some form or another for 40 plus years, so I suppose I know something.
I have logged off of this site and then, obviously, logged back on in order to reply. The only "forgotten" type of option on the login page of this site involves ones password, not username. Some sites have options for a forgotten username.
If you're sure (I imagine you are) that the username "topround" belongs to you, enter that in the username field and then use the forgot password link. Presumably an email with appropriate instructions are then sent to the email address you used when that username was initialized.
It is also conceivable, though unlikely, that this site is experiencing a programmatic and/or database error. However, given that you have now reregistered, albeit with a different username, a site admin should be able to merge your two accounts, delete one or the other, or take no action.
Best.
Hal