I think it's pretty clear what OP wants. He created an account with his email, but forgot the username. He wants to "recover" the username with his email.
The actual answer to his question is: That's not possible, unless he made the email public on his profile and somehow someone finds it (i.e google indexed it).
If it is actually like he said, he remembered the email used for registration, he is able to use it for recovering his account.
If he clicks on Forget password, he will see this.
So he can type either the account username or the email address into the box. He does not know the username, but he knows the email address which is enough.
You can't retrieve your password, but you can set a new one by following a link sent to you by email. You also have the option of setting a new password by answering your secret question.
Username/Email:
How?
https://bitcointalk.org/index.php?action=loginAnd clicks on Forgot your password?
He will be directed to
https://bitcointalk.org/index.php?action=reminderwith the information shared above.
or he can directly visit the page
https://bitcointalk.org/index.php?action=reminder