It depends on the reason you are selling. If you're selling to invest in a more profitable asset, it's actually the opposite... The difficult task is to find something better to invest.
I did this, I sold some of my bitcoins at $2,300 because I have something to buy and it's a real property assets. And I'll use it for making some good monthly income so it's not that an opportunity loss for me. But if you are not planning to invest into something then you can just hold your bitcoins and just see on how much it can grow.