A puzzle looks impossible from the box and obvious from the last piece.

Difficulty is front loaded, and so is doubt. Looking at the picture on the box with a thousand loose pieces in front of you is the point of maximum impossibility, and by the final piece the whole thing looks like it was never hard. Nothing about the puzzle changed. Only your position in it did, which means the feeling of being out of your depth is a poor measure of whether you can do something.

At the box stage of something, do not judge your ability from where you are standing. Place twenty pieces this week.