Don't be fooled into thinking that your success as a stock investor is measured by the increase in the value of your portfolio's holdings. It may be not. What you really want is to make the most returns with the least risk to your investing capital.
In simple term, you may need to fully understand the wisdom of the ancient Chinese's saying in investing: 见好就收. (It means that if there is a reasonable profit for yourself, don’t be greedy to risk further by wanting more, one should take back one's capital and its returns and leave some on the table for others.)
Don't ever let this happen to you!
Read another related ancient wisdom? 3 Ms of Trading – Mind, Method and Money - Part 3