How can I retrieve messages I deleted by accident?

Restoring them from the backup you created before deleting them. No backup = No recovery, sorry.

The only thing you can do, is to ask your correpondent(s) to send those messages back to your iPhone.

Regards

Giulio

Only from a recent backup that includes the deleted messages.

And then - only if you don't sync your messages with iCloud.

From a backup you intentionally made prior.