Changing my Apple ID

I am trying to change my Apple ID to my preferred email. When I enter that email as mine, the software tells me that ID is not available. But that email address and ID are mine. The software is stopping me from using my preferred email and ID and making me use an ID that I hate and that confuses other people.

How can I get Apple to give me back my email address, which is the address I've been using since the 1980s at least. This is infuriating. BTW, the same bad ID is on my iCloud account and I want it all changed.