1) What is one task that you have started innumerable times but just haven’t finished? I'm tempted to say housework, or losing weight. I do have a hand-quilted, almost finished baby quilt meant for a college roommate's daughter. It may be done by the time the daughter goes to college, but I doubt it.
2) What one thing in your life would you consider your biggest mistake or failure? Not overcoming my fear of failure much earlier. My early years were not exactly a hotbed of familial encouragement to try new things; parental response to a proposed new venture almost always began with the phrase "you can't..." As a result, I've spent most of my life not trying anything new or different unless I was guaranteed to succeed. I've finally internalized the truth that failing at a task or project is not the same as failing as a person.
3) What is your general attitude toward failure--do you see it as proof of your eternal inadequacy; simply part of life; something of a learning experience; or as something that shows you’re actually trying to do something? As others have said, some of each depending on the particular circumstances.