A problem that cannot be solved

Have you had a feeling at the start of a project that this is going to be too large, too complicated? What if it is a problem that I cannot tackle? This happened to me a number of times. It was scary. Too many unknowns, a domain that is not familiar and seems too technical, etc. But as the project progressed, things fell into place. The domain with all its technical parts is not that difficult. Things play out well.