Please tell me how I can retrieve my messages that I accidentally deleted

go to Settings > [your name] > iCloud and check the switch position for Messages

If the switch is ON, then you cannot recover your deleted messages.

If the switch is OFF, then you might be able to recover your messages IF you have an iPhone backup that contains the messages you are trying to recover. In which case you'd need to restore your iPhone to that backup. In doing so, you'd likely lose some data between now and whenever your backup occurred.

Assuming you don't house your messages in iCloud, messages are stored on your iPhone backup. Restore that backup to retrieve the messages (and other data) that were present when the backup was created.

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