Examine, investigate and question. You must, because otherwise you will learn nothing.
But when it comes to getting the job done, do it with 100% confidence that it is the right thing to do.
To get something done, you need conviction and faith. A fruitful life will not sprout out of ever-shifting sands, and its home cannot be built upon a foundation of doubts.
Likutei Sichot vol. 18, pg. 154.