The most important thing is not to sell Bitcoin now when you are at a loss. At the next Bull-run, probably Bitcoin's price will be higher than the maximum price reached at the end of 2017.
However, to be sure of profit, you can still buy a Bitcoin now, and when the price reaches $ 20,000 again, you can sell both and you will make a profit.
       There are many ways to make a profit, it's important not to withdraw your investment when you are at a loss.