is there any way to find out all the web sites i belong to?

Answer 1

when you signed up for them you should have made a file with username, password, and security answer for each one. then you would know which ones you belonged to.

but there is no other way to know

Answer 2

Check your old e-mails if you have them. They probably contain your username or atleast a url to the site.

Answer 3

Try to remember all the ones you registered for. When did you first realize you had amnesia?

Otherwsie, search for your name and handles you usually use with google and yahoo searches - might show up somewhere.