How do I I hide my email address??

Did you read my post?

It doesn't matter if you hid it by mistake or not. There is not a way to 'unhide' your address.

The end result is an account created on some website/app using the 'fake' email address.

You will either have to create a new account on the website/app you are working with, or see if you can change your credentials on that website/app.

you don't really...

If you used Hide My Email to create a log in for a website or app, that account is created using the randomly generated email address. Any emails sent will still get to you, but they are relayed. You can delete this from Settings > [your name] > iCloud > Hide My Email, but that doesn't change the account to have your real email address.

So depending on what you are trying to achieve, you could just create a new account for this website/app using your real email address, delete the Hide My Email credentials and pretend that account no longer exists.. Or you could see if the website/app allows you to change your email address, if so, change it to your real email address.