The lesson I have learned is this: as long as neither the setup and nor market conditions have invalidated the trade then I must hold on until my stop loss or take profit stop is hit or the profit target is achieved!
When you first start trading, it is very easy to watch as your stocks drift down and hit your stop loss. However, after you have been trading for some time and have taken some big hits. One becomes conditioned to take small losses in preference to potential bigger ones and this often means exiting before stops are hit. This makes the above lesson much easier said than done!
In addition, I am discretionary trader by nature so I tend to find difficulty with the mechanical aspects of my trading. Yet I am working hard to combat these weaknesses and continue to look for ways to refine my system so that I can achieve my target of 1 Mil in 5 years!
Good luck and good trading!