How to see deleted messages on iPhone 11?

The only way to recover deleted messages would be to restore your iPhone with a saved backup. This process is time-consuming so you might want to consider if it is worth it for a couple of messages, and it would be dependant on you having a backup that has these text messages saved, so if you don't back up your phone regularly (I personally have mine done automatically which you can learn about here) then this process might not work.

You may also find this link helpful in regards to restoring your device from a backup.

Restore your iPhone, iPad or iPod touch from a backup – Apple Support

I would advise you to set up a passcode which means your girlfriend won't be able to access your phone and delete messages in the future.

The only way to perhaps recover them is to Restore a backup made before deleting the messages but you will lose any info made after the backup.