The question itself is the answer
PANIC.
Everyone knows cryptocurrency market is highly volatile and risky.
And most of the people do emotional trading.
They sell when it is crashing. And suffer from the loss. They panic and sell in the loss.
What is the point in selling at low?
One must wait for the market to turn green and bullish.
The way to stop the panic is to not engage in panic-inducing activities such as day trading.
It is like the old joke; a man goes to the doctor and says my arm hurts when I put it overhead.
The doctor says dont put your arm overhead

If you are in need of urgent money than selling is OKAY but for profit,
HODL is the best.
You can stop panic selling by improving your self esteem. Always have courage to face all of the risks because facing risks can improve you. Do not afraid to fail because failing is part of becoming successful person. It is normal to fail sometimes, just do your best to overcome different trials. It is true that trading is difficult job but I am sure that it will be easy for us if we fully understand how trading works.