This article discusses some strategies to recover deleted Facebook messages.
How to Recover Deleted Messages on Facebook
When you use the Facebook app's Activity Log to delete a post, you can retrieve it for up to 30 days. However, this won't work if you delete a post directly from your Home feed. These steps explain how to do this in the app:
Open your Facebook profile and tap the three dots menu below your photo.
Select Activity Log > Activity History.