Hi WSTTHMSS,
According to your description, I’d like to confirm the following questions to look into the issue:
1. Have you changed your email address from mydomain.onmicrosoft.com to yourcustomdomain.com?
2. Do you mean you cannot sign in to Office 365 using mydomain.onmicrosoft.com?
If you have changed your email address to your custom domain, you only can use your custom domain to sign in to Office 365 rather than the initial account you registered.
If my understanding doesn’t cover your question, please post back to correct me and provide me with more detailed information.
Thanks,
Tracey Wang