M ex-wife has somehow gained access to my iPhone

To keep your Apple ID account secure, see the following Apple articles:

Manage and use your Apple ID

Manage and use your Apple ID – Apple Support (UK)

See who has access to your device or accounts

See who has access to your device or accounts – Apple Support (UK)

Recognize and avoid phishing messages, phony support calls, and other scams

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Phony "tech support" / "ransomware" popups and web pages

Phony "tech support" / "ransomware" popup… - Apple Community

Identify and filter junk mail in iCloud

Identify and filter junk mail in iCloud - Apple Support

If you think your Apple ID has been compromised

If you think your Apple ID has been compromised - Apple Support

Identify legitimate emails from the App Store or iTunes Store ->

Identify legitimate emails from the App Store or iTunes Store – Apple Support (UK)

Set up iCloud Keychain

Set up iCloud Keychain - Apple Support

Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 

Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

See also the following Apple articles:

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support

Authorize or deauthorize your computer for iTunes Store purchases

Authorize or deauthorize your computer for iTunes Store purchases - Apple Support 

Get a verification code and sign in with two-factor authentication

Get a verification code and sign in with two-factor authentication - Apple Support

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to see where you're signed in - Apple Support

if you see devices that don't belong to you, delete from your account.

Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch

Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch - Apple Support

Block unwanted callers in FaceTime on iPhone

Block unwanted callers in FaceTime on iPhone – Apple Support (UK)

Avoid unwanted calls on iPhone

Avoid unwanted calls on iPhone – Apple Support (UK)

See also ->  Share your screen in FaceTime on your iPhone or iPad

Share your screen in FaceTime on your iPhone or iPad - Apple Support

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Everyone can see what's on your screen until you stop sharing, but they can't control anything on your device. If you get any notifications while sharing your screen, others won't be able to see them.

When you're sharing your screen, others won't be able to see content that requires a subscription, a free trial, or a purchase or rental to view. 

Checklist: If you want to make sure no one else can see your location

Checklist: If you want to make sure no one else can see your location - Apple Support

There is no way anyone (not even the FBI) can gain access to your phone unless they have physical possession of it while it is unlocked, or if they know the passcode. If you think this has happened restore the phone to factory settings→Restore your iOS device to factory settings - Apple Support and do not restore a backup.

It’s possible your Apple ID was compromised; for that see→If you think your Apple ID has been compromised - Apple Support