When I talk in game I can see it in the chat box. However I don't see the text appear over my character in the main playing screen. I must have disabled it but I don't know how to re-enable it. Where can I access those options to make my chat appear over my character's head?
This problem has most likely been fixed. The most recent patch notes contain the line
A bug where chat wasn't appearing above player's heads has been fixed.
As there is no way to disable chat appearing over your head in game, a bug was the most likely cause of the problem.
Your private chat is turned on; just turn it off.