Two things come immediately to mind.
The first is that your password might be too long, or contain characters other than numbers or letters (which are case sensitive). Passwords should be between 8-12 letters and/or numbers long, and if you reset a password using the previous password, it will “blank” the field and force a new reset.
The second is that you either have not allowed third party cookies, or you have a cookie string which is corrupted. If you are certain your password is fine, then I suggest you do this. Clear your browser cache and history, and close it down completely to ensure the deletions take place. Restart your browser, and log in to AFF using your exact email and password. The site will set new cookies, which should then allow you to log in.
Just to clarify, you cannot register more than once here at AFF. The system will not allow it.
If you are still having problems with the log-in, let us know, and if possible, give us as much detail as you can (without posting your email address or password publicly, of course!) so we can better assist you.