Apple ID recognises email but then it does not

I often have trouble with my Apple ID account. It works for a while but one day will say 'some account services require you to sign in again'. My heart sinks when this happens, because I know it will not recognise the login details, even though I keep them written down.

This time, I'm being sent round in circles. If I use the Apple account identification process, it accepts my email and gives me an ID which is a name I created. I then go to the sign-in (which it will only accept the name mentioned and not an email) and it lets me sign in, but then it says: 'this Apple ID is not an email'. I enter the same email I used to reach that page and it says: 'the email is already associated with an Apple account'!!! But I used that email associated with the account to get to where I am!

It seems the only option I have is to use a new email, even though it accepted the original email to reach the point of accepting a sign-in. WTF is going on?