Forgotten username

Customers sign in with their email address, so there is no separate username to forget. Forgot username on the sign-in screen explains this and, if the email entered is on an account, sends a short reminder to it. As with the password reset, the reply is the same whether or not the address is registered, so nothing is revealed. In short: sign in with the email address the account was set up with, and use Forgot password if the password is the part that has been lost.