I would say always put stop loss even when the market seems bullish , 15% means you didn't use stop loss and I would sell it if it happen to me because stopping loss in any point is better than losing more , Try to trade non emotional and forget about the past .
Sometimes people dont set stop/loss at drops like that. It would be foolish to trade off all your assets at a loss because you are just emotionally moved that you have lost your money. What people dont realize is that they have not lost their money but the value of the asset has dropped. But it can happen and is nothing to be panicky about. Same in other markets as well.
Crypto being very speculative is subject to these risks and one must be aware of them before getting in the market. No use panicking but remaining calm is what they must try.
It's really important to be smart and not put everything in one basket because this market dropped could make you emotionally and panic.
You are correct, we don't loss money even if the price will dropped as long as we stay strong and hold our precious coins.