OK so I'm watching season 5 episode 12, where Clark tells Lana about his secret and then the second time doesn't tell her. Can anyone pleeeease tell me when he is really going to tell her again? I'm so curious!!

Truespock: yeah you are correct that that CK doesn't tell Lana about himself after that one time. However, she tricks him -- thus finding out for herself completely -- in Season SIX not Season eight.

The episode in which she finds out was 'Promise'; i.e. her wedding day.