What is the one thing that I should learn which may benefit me one day?

I will advise you that never mourned or grieved on if you missed a getable thing or chance, as yet so near yet so far, or something you lost in your hand, or your all efforts went in vain. Move further & forget, proceed further and never look back. A lot of successes are in your way but go ahead. To clarify my point I will narrate a short story.

A hunter set a trap to catch birds. Coincidentally, a tiny beautiful nightingale got stuck in it.

The nightingale said to the hunter! You have hunted many large animals. What am I in comparison with them? A very little bird, have a bit of flesh in my body, it will not even fill your stomach, what will you feed your family? But if you set me free, I will give you three pieces of advice that will be very useful for you, if you follow.

Hunter was already impressed with its beauty, now he became more curious, what is she going to say. He agreed and said to the nightingale. First, advise me, then I will let you go.

Nightingale, OK. First, Don’t believe what can never happen.

Hearing this, the man let her go of the hand, she sat on the wall in front of him. Then she spoke.

Second, "Don't grieve what you have missed, doesn’t matter how precious it was."

And then she said. You made a big mistake by leaving me because I have a very rare pearl in my stomach. If you took out this pearl, you would have gained a lot of wealth and it would be enough for many generations to come.

When the hunter heard this, he mourned and became upset about what he did. Then he tried to catch her again. But the nightingale flew away and sat on the high branch of the tree and spoke. You forgot my advice, not to believe anything that is impossible. I am a nightingale that weighs a few grams and has a big pearl in its belly weighing ¼ kg. Is it possible?

Secondly, you became sad, why did you let me go. Anyway, It is useless to give you any new piece of advice. You did not follow the first two, so what about the third one? ”Saying this, the nightingale flew away.

In the end, Don't grieve what you have missed, doesn’t matter how precious it was.

Here I’ll quote a most beautiful saying of Holy Prophet (PBUH)what helps a person not to disappoint in any case.

“Knew that what has come to you could not miss you and that what has missed you could not come to you. (Sunan Abi Dawood: 4699)