Tracking my phone

• Update to iOS 14.7
• Change to a stronger password for your Apple ID
• Use 2 factor Authentication => Two-factor authentication for Apple ID - Apple Support
• Make sure you have at least 2 trusted phone numbers for 2 factor authentication (note: landline can also be used)
• Don't share you locations with your ex => Location Services & Privacy - Apple Support

Go to Settings/[your name]/Find My and remove any devices that are listed as sharing your location. If your ex knows your Apple ID and password change your password→Change your Apple ID password - Apple Support