Are you sharing an AppleID? If so, this is the cause of your problem. iPad and iPhone are designed to be single-user devices.
If all your devices are sharing a common AppleID for iCloud and other Apple services, you will encounter all kinds of issues with unintentional and/or unwanted sharing of sensitive/private information (including Messages).
The Golden Rule: Don’t share an AppleID between users. Each user should have their own AppleID for their own exclusive use - linked to their own primary email (and secondary/recovery email addresses and trusted telephone numbers).
You would be best advised to separate your digital identities and configure a new AppleID for each user
After separating your identities, you can continue to selectively share purchased Apple Services, Subscriptions, Apps and Media using Apple’s Family Sharing features.
Family Sharing - Apple
Set up Family Sharing – Apple Support
Share App Store, iTunes Store and Apple Books purchases with Family Sharing – Apple Support
How to create a new Apple ID – Apple Support
https://www.imore.com/how-set-up-family-sharing-iphone-ipad