An improvised moment becomes unrepeatable before anyone knows whether it was good.

Improvised moments end before anyone can judge them. The thing is offered, it lands or it does not, and there is no chance to weigh it first because the weighing would have destroyed the conditions that made it possible. Spontaneity and evaluation cannot occupy the same instant.

If you want more of those moments, you have to accept the ones that fail as the cost of the ones that work. Practise speaking slightly before you are certain, in low-stakes company. Most of what you produce will be ordinary and some of it will be better than anything you could have planned, and there is no version of this where you get the second without the first.