Sleep. Once it is lost, it can never be reclaimed. Speaking of which, I need to start getting more sleep.
This aspect of sleep is similar to time. Once I pass up on an opportunity, I can never get it back. Luckily though, time is a bit easier to negotiate with. I can wait it out and continue to actively seek similar opportunities elsewhere. However, there will always be regret when great opportunities are lost. I think regret serves as a teacher in such cases. Regret is a great teacher. That goes without any exaggeration.
I wouldn’t want to repeat a class with her. It would be too heart-wrenching.