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I am sorry you experienced frustration in printing this model.

Nuts are measured on ID (inner diameter) and bolts are measured on OD (outer diameter) of the shaft. That way, a 1/2" nut goes with a 1/2" bolt. It is easy to buy matching bolts and nuts. This is exactly the same for metric bolts and nuts. An M12 nut is 12mm inner diameter to fit an M12 bolt (12mm outer diamter).

Under your measuring scheme, a 3/4" OD nut would fit a 1/2" bolt. That doesn't make a lot of sense. Unless you also measure bolts by OD of the head, but the head on the bolt is typically smaller than the nut, so that wouldn't match either. And what about round head bolts?

The nut in your picture is a 1/4" nut. There is a model for 1/4" nuts, but the model you printed was designed for 1/2" nuts.