Yes you are right, I already checked the Facebook rules when it comes to message and it says once you deleted your messages you cant undo that anymore. So what I need to do now is to accept that I cant get it anymore even though there is some important message there. So guys, if you are planning to delete your conversation in Facebook to remember to save your important things in that conversation before totally deleting it.
I believe that this is untraceable since you've deleted it initially in the first place already. But if ever you really need to retrieve those deleted messages, maybe you can try contacting customer support and ask them if its possible. Hope this helps!