I can't sign in or out of iCloud. Forgot the password to an email address I no longer have access to. What next.

I can't sign in or out of iCloud. Forgot the password to an email address I no longer have access to. What next? From what I'm reading in the forums, once the email address is set up it cannot be changed. True? If so, um, stupid.

Message to log in to iCloud every time I reply or forward an email. Have to indicate to ignore every time. Annoying.