While it seems hopeless, you can still try your luck! Macworld compiled these four ways to retrieve deleted texts on iPhone that you can try:
Retrieve deleted texts on iPhone: phone provider
First option you have is to contact you phone provider. Some, although not all, will keep a record of your text messages, and you are entitled to access them. It may even be as easy as logging in to your account online. For some it requires you to contact customer services.
Retrieve deleted texts on iPhone: from iTunes backup
This works only if you automatically back up text messages.
1. Connect your iPhone to the PC with which you sync it. iTunes should open (if not, open it up manually).
2. You should see your iPhone in a box, top righthand corner. Select it.
3. Now choose 'Restore backup'.
4. All the data you previously backed up will now replace the data on your phone. It will take a few minutes. As long as you haven't backed up subsequent to deleting those text messages, they should now appear in the normal messaging list on your phone handset.
Retrieve deleted texts on iPhone: from iCloud backup
Although iCloud doesn't back up SMS in all territories, and for all phone operators, it's still worth a try.
1. Login to https://icloud.com with your Apple ID and password.
2. Click on Text Messages (if it isn't there your messages haven't been backed up).
3. If it is there searched the messages to find the ones you need.
4. Now go to your iPhone and choose Settings, iCloud on your iPhone.
5. Turn off Text Messages (or ensure it is turned off). I know this sounds counter-intuitive but stay with me people.
6. You'll see a pop-up. Select: 'Keep on My iPhone'.
7. Now turn back on text messages.
8. Tap 'Merge', and wait. After a few minutes the deleted text messages should appear back on your iPhone.
Retrieve deleted texts on iPhone: use a third-party app
If neither of the options above has worked, you can look for third-party apps that appear to have a good online reputation.
Check out these top text-messaging etiquette tips shared by Macworld to avoid annoying others when you send text messages.