You have to call Playstation customer support at 800-345-SONY, and provide adequate proof that you created the account. Knowing the date-of-birth and security question answer that was registered for the account helps a lot. Sometimes having the serial number of the PS3 that the account was created on works too. If you have billing information associated with the account, knowing that may work also. If you can prove to the satisfaction of customer support that the account is yours, then they will tell you what the sign-in e-mail address is, or change it to a different one that you have not forgotten.