The greatest feeling of accomplishment you will ever feel will be rooted in doubt. The man who is sure of himself in accomplishing the mundane feels nothing. It is the man who dares to pick up his sword as he charges into the unknown adventure who feels the adrenaline that reminds him he is alive.
Why do we have doubt? If there was not some evolutionary advantage to having it 40,000 years ago, we would not have doubt today.