How do I prevent someone from accessing my iPhone and Apple ID?

First, check for a remove any profiles not installed by you. See https://support.apple.com/guide/iphone/install-or-remove-configuration-profiles-iph6c493b19/ios

Then you need to check for and remove any unknown devices from your account and secure your account. Follow all instructions at https://support.apple.com/en-us/102560

You’ve changed your Apple Account (formerly Apple ID) passcode, correct? If so, it’s unlikely your email and/or iPhone are accessible to anyone but you. An exception would be using very simple or previously used passcodes that your EX could easily guess.

Some things to get started:

1. Delete any device profiles that you find installed on your iPhone: https://support.apple.com/guide/iphone/install-or-remove-configuration-profiles-iph6c493b19/ios
2. Check your Apple Account device list to find where you’re signed in (https://support.apple.com/en-us/102649)
3. Change your Apple Account password (https://support.apple.com/en-us/101567)
4. Follow through with Apple's Safety Check for an iPhone (https://support.apple.com/guide/personal-safety/safety-check-iphone-ios-16-ips2aad835e1/web)
5. Change your password for all messaging apps, email accounts, and social accounts. Enable multi-factor authentication for all of them.
6. When changing passwords, use a long, strong, and unique password for every account (https://www.nist.gov/cybersecurity/how-do-i-create-good-password), store them in Apple Passwords app (never reuse a password)
7. Use the list view of the iOS App Library (https://support.apple.com/guide/iphone/find-and-use-your-apps-in-app-library-iph87abad19a/ios) to review every app on your iPhone, remove all apps that you do not recognize or use regularly.
8. Check the account settings for all of your email accounts and remove any forwarding rules that you find
9. Check your sharing settings in all social apps that you use.