I'm pretty sure the only way you can do this is using the E-mail you registered for the first one. If you don't remember your E-mail, that's too bad :/
But if you do, you can do one of two things.
1) Check if the email still has the verification email. It should have your username in it. 3) Send a lost password report. It should send you an email asking to change your password. But I'm like 30% sure that it's in there.