How to protect my Apple ID?

The fact that she is the account holder for your cellular plan does not give her access to your Apple ID account. If she has the password for your Apple ID, see: Change your Apple ID password - Apple Support

The following may also help: If you think your Apple ID has been compromised - Apple Support

Bullythekid Said:

"How to protect my Apple ID? I meant to say my girl friend is the account holder for our 3 cellphone lines but she has also been changing thinks in my Apple ID"

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What kind of a friend would do that?

Consider Modifying your Crdentials:

I recommend that you modify your credentials, starting with a new password. Don't share this information with anyone whomsoever.

• Modify: your login credentials (Using Both a New eMail and a New Password for each and all sites)
• Associate: a different method of payment to your account
• Enable: Two-Factor Authentication for Apple ID and for all other accounts that have such an applicable option