Not sure if this has already been mentioned, but if you already have your gamertag linked to an old EA account and you dont remember E-mail/password. Go to Login, Forgot password, and then enter your gamertag. It will send an email to whichever one is linked to your tag, then you just have to figure what email it is and make a new password.
This is what I did, because I was having the same problem.
Available on iTunes, Windows Phone, YouTube, and more!