What would you regret not fully doing, being or having in your life?

I would definitely regret for the blunder that i have made in my entrance exam in my 12th standard and still i wish to do it again but i know that past is past and it is not here so i think of it again and again in my present stage. I think i am still young to think of the tasks which are not fully done, being or having in my life. I am a student so i definitely think of my past studies and not other thing lol. Anyways nice topic to discuss. Keep on adding interesting topic which can encourage me in answering more and more too good topics. Keep smiling and happy living :)