When your business encounters the inevitable pitfalls, detours and slowdowns, you will be the one others will look to for reassurance. You will then have to call on the reserves of your own faith in your project to bring back the momentum. This will happen when your own faith is most challenged. If you do not bring a healthy and respectful appreciation of the challenges you will face when building a business, you'll be unable to withstand the difficult times. I have met many would-be entrepreneurs who have foolproof ideas for businesses or inventions but who will never succeed because they do not have a sufficient level of doubt.
Nothing is foolproof.
You have to allow yourself to clearly see and anticipate problems. If you believe you cannot fail, then you won't be emotionally prepared to deal with the inevitable setbacks. You have to recognize that there will be great difficulties, and you need to be able to adapt, regroup and re-energize. You must challenge yourself and be willing to take a clear-headed look at your plans, abilities and the strength of your competition so that you can respond. Great doubt is a part of a success, but it is not necessarily an indication of failure.