Someone else has accessed and linked up to my account how can I find out who and stop it

Change the password to something else.

appleid.apple.com

Also make sure there are no other unwanted email addresses linked to your Apple ID or phone numbers.

You might want to make a note of any unwanted email address and phone numbers so you can report them to Apple.

reportphisihing@apple.com

I didn’t post this question but I have a crazy ex who continues to access my account and changes my password, how do I fix this issue?

Add 2FA.

This should prevent them from being able to change your password without also having access to a trusted device.

Two-factor authentication for Apple ID - Apple Support

Carolyn,

I didn’t post this question but I have a crazy ex who continues to access my account and changes my password, how do I fix this issue?

Hi ...

How do you know this for certain ??