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Another way to hide your messages is to archive them in your Notes app. You can do this by unlocking the note and sharing it with the person. You can also use screenshots to hide multiple messages. This way, you can easily hide your messages from a single contact. It will not be visible to your girlfriend, but you can still keep them safe. You can use the screenshots to keep them safe and secure.

See also - https://foxnewbrand.com/hide-text-messages-on-iphone/

Yes, this is a good way to hide old messages using the archive. Also, you can manage data and empty the mailbox.