Hacked account.
My ex girlfriend is using my Apple ID. I cannot log into my Mail on my device. I need this changed please
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You might want to consider changing your password and making sure it is a strong one. You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.
Link to contact Apple Support in the article.
Apple ID has been compromised.
Note the restrictions in the article.
What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.
Apple ID -Two-factor authentication
Two Factor Authentification availability.
2 Factor Authentication - How to generate a recovery key
Apple ID - Using app-specific passwords.
Apple ID - Two Step Verifications FAQ