I do believe there were instances of soldiers breaking down in ancient and medieval battlefields. However, people those days didn't recognize what it was and took it for cowardice, which is why it is barely recorded. But it has to be said: death was a much more closer thing to people back in the day then it is now. In fact, we live in a society where we dread death and want to push it away, while in the medieval world, people lived very close to it and it was much more common and real to them than to us.