Is buy and forget a myth?

We are told to buy in a good company and forget.
There are many examples of companies that one bought in 1990 and just kept through ups and downs. Pidilite for example, was 150 in 2011 and today is ₹2587.
In fact, we are told that when a good company is temporarily in shani dasha, that should be the time to add more. One very famous example is that of Warren Buffet adding more of American Express when it was faced with the Tea Oil scam.
But then another voice tells you to sell a a share which is plummeting. Afterall, how long will you keep faith in a sinking boat?
In fact, I got out of HDFC and HDFC bank when they would not move. Its alright to advise patience, but when other stocks, including in the financial sector itself, are jumping with joy, why would you keep a morose stock. This is the theory of not remaining married to a stock.
What I have understood is that there are many theories, but you have to choose which suits your temperament. There are people who have THAT sort of patience. I am not saying that patience is not required for a long-term investment, and one should be trigger-happy. What I am saying is that at times it may be difficult to say that the blip is temporary or it is really indicating that the company has really fallen on bad days. Suppose somebody had stuck with a Kodak, after all it was a great company! May be one would say that an investor has to be alert. But doesn’t it amount to saying the same thing, that is, that there is really no investing and forgetting?

All of this is a preface for sharing an article by Devina, I have chanced upon on twitter.