People often think only about starting, they put all their focus on knowing when and how to start. But I've found that if you don't also know when to stop, you may have lots of problems in life. Imagine that you went to a Cinema to watch an amazing movie, and you keep on watching for long hours, and the movie doesn't come to an end. No matter how interesting it is, you'll want it to stop, so you can go home.
Thus, a movie producer has to understand that while he needs a mind blowing start of a movie, he also needs to know when to stop. You always need to know how to stop. When you relate this to trading markets - you'll see that it's not just about you knowing how to spot a good opportunity. You need to know when the opportunity isn't there anymore, and you stop that trading session, so you can trade again, when you see another great opportunity.
The problem with most traders is that they don't know when to stop. In fact, some of the biggest losses are made after a series of successful trades. If a trader knows when to stop, he will know when the market wants to take back all the profits he has made. Hence, he stops, and trades again another time. The truth is that knowing when you should get out or stop is paramount in life.
Even when you're learning a skill. You cannot be working for long hours unending, without stopping for a short break, or stop to go have a walk around. When you continuously exhaust yourself, doing and doing; and not stopping for some time - you'll notice that your productivity declines. Those who make themselves into masters in their fields, know when to work and when to stop working, to rest and relax.
Hence, you need to understand that there's need for balance. If something isn't going according to your plans, rather than just continuing doing that same thing, it's more helpful for you to stop. Stop for a day or two, and really think about the reasons why you're not doing it right. The answers tend to come to you, when you take those stops or short breaks.