You only have one choice: call Support and go thru their "account verification" process: Like you, I had an account once that I could not remember the password to and the email was dead so I could not get a password link. I called support and tried to do this, but could not answer all their questions.
After awhile, another poster here took pity on my plight and encouraged me to try again and use the email version of the verification form. I contacted suport (I think you can also get it from the WLID site) and got an email sent to me. It asks a pile of questions like: name, address, phone, card nos, etc. Even what your ISP address on your computer! It also has a text box for more info. Put in friends' names, your GS, even some acheivements...hopefully, these will offset and questions you can't answer!
In the end, after 8 months and 6 forms, I finally was able to send a password reset link to an email that I wanted and was able to access that account. Basically, in the end, if you can't answer their questions to their saitsfaction, your account is gone sure enough. Use the email form: you can take time to research the answers!
And by the way, it is silly to think Xbox knows your "secret" password: after all, if thousands of support people could access your password, can you imagine what would happen; i.e, MS does not keep up with passcodes.