Stop sanding when it feels smooth with your eyes closed. Some finish lines you feel rather than see.

Your hands detect things your eyes cannot. A surface can look finished and still catch slightly under a palm, and the hand notices the ridge the eye slid past. The point is also that done is a condition you can check rather than a feeling you wait for.

Next time you are finishing something, close your eyes and run your hand across it. Work only where you feel a change, and stop when you do not. Try to find the equivalent test in whatever else you make, like reading a paragraph aloud instead of rereading it.