One of the great "secrets" of successful people is discipline and it doesn't make any difference whether it is manufacturing, processing, servicing or investing in the stock market.
Before you can have that discipline you must have a successful plan and stick with it. If the method you use does not work or results in smaller profits it should be abandoned and a better one found. For the average investor the plans laid out by Wall Street do not work and over the long run you will lose money. Actually you will make a very small percentage, but the return will be mitigated due to ongoing inflation. The great majority of investors believe that an annual return of 10% or more is to be expected when actually it is much less and there will be periods when there will be almost no return at all.
Returns can be increased greatly if the investor will learn not to follow the 3 great lies of Maul Street. They are Buy and Hold, Dollar Cost Average and Do Research. These lies have been told so often that they have become conventional wisdom.
During 1998 and 1999 the price appreciation was fantastic. If you check back in history you will find this was an aberration. Folks still think that was "normal". The actual norm is about 16 to 18 year periods of bull markets followed by bear markets with many 4 year cycles of ups and downs with that 16-year time frame.
Think back to 2000, 2001 and 2003. During that time did your broker ever call to tell you to sell? About 98% of brokerage company recommendations were to Buy. Many folks lost 50% to 80% of their savings. That alone should have turned on the light bulb in your head that either these guys are stupid or they are lying to you.
There is a "secret" to investing and it is one word - Sell. You must have to discipline to remove yourself from losing positions. During the worst part of that 3 years we saw many stocks drop 50 to 90% and other companies go out of business. It you have placed a limit to the amount of loss you would take you would have a lot more money today. Why do you want to wait for your stocks to drop 30, 40, 50% or more when you could have placed an Open Stop Loss Order with your broker to sell you out if your stock dropped below a certain price? Maybe 10%, hopefully not 20%, but even that is better than a huge loss.
In many cases brokers try to talk you out of selling, but your discipline will require you to be firm. You must protect your money; insist on protection of your investments.