Well, there is no foolproof way to list out all the sites you have been registered to. There are numerous sites and forums available all over the internet. Some of them are popular, while most of them are not.
If you have been using Chrome. or any such specific browser for connecting to your sites - there could be a possibility of being able to find out the sites you have registered yourself on. I am making that statement based on the assumption that you have been saving the passwords on chrome when prompted. On chrome, you may follow these steps.
1. Launch Chrome on your PC, or Laptop.
2.Click the options ( the three horizontal lines at extreme right).
3. Locate Settings and click on it.
4. Click on Show Advanced Settings at the bottom to bring up additional settings.
5. Locate Passwords and Forms. Click on the Manage passwords link.
6. It will launch the page wherein a lot of information with username, site name and password in asterisks.
That should give you an idea of the sites you have signed in to.
KnowEm.com. is another site that can be used to find the sites you have signed on. You just need to provide the usernames you regularly use on the sites and the KnowEm lists the websites indicating whether you the username has been registered on a particular website.
There are many apps or sites that work the same way. It is dependent on whether you have saved the login information. A few examples are Keeper, Dashlane3, LastPass, Sticky Password Premium, KeePass etc. But, since you are searching for an option to find the sites you have joined back in time, maybe these options would not be applicable for you.
Live....and Let Live!