I do not think that you understand the concept of 'investment'.
When you start to invest into something, you must have an actual goal towards it, whether you view it as short or long-term for its value to mature. Cryptocurrency, bitcoin to be specific, tends to have a high-volatile price due to many factors (such as: free from government regulation, media that attracts new investors each day, etc.) and investors have the prerogative of selling their investments whenever they reach to the point of profit. Again depending on the goal of an investor, selling bitcoin may be the right choice depending upon the circumstances on how you acquired them- never sell your investments to a price lower from what you acquired them.
Selling your bitcoin at the right price and still you can gain profit is not a stupid decision. Acquiring your btc at a higher price and you plan to sell this time where price is still low, that is where I can say a stupid decision. The best thing to do for now is just to hold your bitcoin even if it take a longer period of time to recover then do it. This is the only option for now to avoid bigger loss from your investment.
It all boils down to the right time and opportunity when you are going to sell your coins. Like what I mentioned, the rule of thumb is that you should never sell your investments to a price lower from what you acquired them (exemptions- extreme circumstances that would require you to sell due to financial struggle, etc.). Depending on the goal of an investor, he/she may sell them if they see that the market is right or for long-term which you may hold for a while and sell it after 5-10 years.